Marco and "Let's do it," Rachel's catchphrase. He just wanted to say it first.
Jake and "Charge!"
In an interview, K. A. Applegate stated that Rachel's "Dark and stormy night" and "I always wanted to write that" lines were Applegate speaking to the readers.
I Ate What?: Inverted. Ax eats all kinds of stuff (cigarette butts, engine oil and a paper plate to name a few) but it doesn't really bother him. He can't see what the others are getting so upset about.
Cassie: Were the nachos good?
Ax: They tasted of grease and salt. Plus, there was another flavor that reminds me of some delicious engine oil I tried once. Oil. Oil-luh.
Jake: Ax...You know how I mentioned you can't eat cigarette butts or dryer lint? Add engine oil to the list.
Identity Impersonator: In later books, the Chee start doing this for the kids when they have to go on long missions.
Idiosyncratic Episode Naming: The regular series books are all in the form of "The <Noun>." Likewise, the three prequel books are all in form of "The <Species of the Person the Book Focuses On> Chronicles."
If I Wanted You Dead...: The second-last book. Not only could Tom have fought them with his battle morph (jaguar), but he could've conceivably called in every Hork-Bajir-Controller on the planet and taken them down.
If You Kill Him, You Will Be Just Like Him: When the war ends, Jake refuses to kill Visser One (previously Three). Tobias, enraged, demands to know why, claiming that Visser One was the one responsible for the entire war. Jake replies quietly that they "don't kill prisoners."
The Visser immediately mocks his hypocrisy; Jake has just killed seventeen thousand unarmed, helpless Yeerks. Not to mention he and the others had blown up the shopping mall to take out the Yeerk pool beneath it, which killed thousands of unhosted Yeerks and hundreds of innocent people.
Rachel is about to kill Tobias' captor Taylor, but Tobias urges her: "Be Rachel. Not her."
I Know You Are In There Somewhere Fight: Rachel tries to talk Tobias into controlling his Deinonychus morph, first by appealing to his humanity. But when that doesn't work, she grudgingly tries to accept the fact that he's a bird now.
I Know Mortal Kombat: Marco tries to use knowledge from video games to drive a truck and operate a tank. He doesn't kill anyone (trash cans excepted), but he does scar them for life.
Impact Silhouette: In book #25, The Extreme, the Animorphs (as Polar Bears) are being chased by Venber at the Arctic Yeerk base. Marco stops quickly and a Venber misses him, slamming through a steel door and making a vague Venber-shaped hole in it. Marco even calls it a "Bugs-Bunny-runs-through-the-door kind of hole."
Impostor Exposing Test: In The Unexpected, Cassie, hiding from the Yeerks on an airplane, tries to pose as a passenger. The Yeerks, knowing she's the only one on the plane who hasn't been affected by their paralysis-inducing phlebotinum, ferret her out by shooting everyone with low-intensity Dracon beams and seeing who flinches.
Informed Judaism: Jake and Rachel are presumed Jewish (most of their parents and siblings have Hebrew names, and Rachel flat-out states "My dad's Jewish," when they visit D-Day. Dialogue from #21 and #31 allow us to conclude that their dads are brothers.), but nothing of their beliefs or holidays is mentioned beyond that.
Well, according to Ax, Jake's family says a short prayer before dinner, but we don't know which one.
In Harm's Way: Rachel. Also everyone else besides Cassie to a lesser extent.
Insane Admiral: Yeerk Vissers in general. Visser Two from The Deception is utterly bat-shit crazy, and gets bonus points for taking over an actual admiral.
The Internet Is for Porn: Actual pornography is implied once or twice, but this being a kids' book, we get a more toned-down version: Marco uses the internet almost exclusively to look up scantily clad girls.
Interspecies Romance: Aldrea and Dak Hamee, Elfangor and Loren. Also, Tobias and Rachel, sort of (Tobias is stuck as a hawk, but he was originally a human and thus identifies as one. Also, while his original form is genetically fully human, his father was actually an Andalite in human morph, so it might be vaguely interspecies even if he wasn't a hawk most of the time.)
I'm Not Doing That Again: Frequently used as a catchphrase, but once very serious, when the kids morph ants - the ant morph has some very disturbing side effects.
Cassie also says this when she leaves the Animorphs for a brief period, referring to a particularly grisly battle.
Ax also drops this one in #31, after torturing Chapman.
< I will gladly fight this Controller, and even, in fair battle, kill him, but I am not a torturer. >
Andalites are the "good guys" and Ax often points out the immorality of human actions for Aesop effect - for instance, he states that human wars are often pointless and needlessly cruel. However, setting aside the horrifying ruthlessness of the Andalite military, Andalite citizens themselves are no saints. They have a variety of distasteful flaws, such as extreme arrogance. Most notably, they intensely hate vecols - the disabled, who, they believe, should ostracize themselves from society to preserve themselves from the shame of being "incomplete" and therefore worthless. Most notably seen when Ax strongly objected to meeting Mertil, the marooned Andalite fighter pilot from #40, The Other, and Mertil's companion Gafinilan opposed the other Animorphs meeting him as well, on the grounds that subjecting Mertil to the public eye would be shameful.
Hork-Bajir used to be a species of Innocent Aliens. It wasn't that they were pacifist - they simply didn't understand the concept of fighting. The Yeerk-Hork-Bajir war, however, changed that. Hork-Bajir are still simple pacifists in nature, but they're far from the Innocent Aliens they used to be.
Also, the Howlers, who are, from a certain standpoint, the most innocent species that the kids meet. Unfortunately, for Howlers, "fun" means "killing the shit out of everything they see." Howlers are a species of genetically engineered killing machines. It's just that they don't know that killing is wrong, and Crayak preserves their naivete by altering their collective memory.
The Pemalites may fit this trope, though the kids never meet the Pemalites themselves, as they were killed by the Howlers thousands of years ago.
It's Up to You: Combines with Comes Great Responsibility. The kids are the only ones who know about the invasion who also have the power to resist it. If the kids give up, Earth is doomed. This doesn't make them happy.
Jerk Ass: Marco and David, though Marco's generally regarded as more likable about it.
Just for the Heli of It: Tobias and Rachel (in bird morph) are following a woman by latching on to a taxi. Then Tobias, for some reason, gets it into his head that they'll lose her if they stay on the taxi, so he flies up to a nearby helicopter and grabs on to the skids, a dangerous act and, as it turns ot, pointless, since the helicopter ends up landing at the airport where the woman (Visser Three in morph) was going.
Kangaroo Court: Visser One is tried as a traitor and an Andalite sympathizer, but in actuality, Visser Three set up the whole thing and convinced the Council to go through with it in order to usurp her position.
Kicked Upstairs: Tom's second Yeerk, who is improbably promoted to chief of security despite being the last Yeerk qualified for the position. It's implied that he's promoted simply because the job requires working closely with Visser Three, which Genre Savvy Yeerks recognize immediately as a death sentence.
Kick the Dog: Literally. To keep alleged Friend to All Living Things William Roger Tennant from recruiting for the Sharing, Marco morphs his stepmother's annoying poodle and torments Tennant every minute possible, with the namesake result televised live.
Kill the Poor: One of the Megamorphs books begins in an alternate universe where the homeless "are rounded up and shot."
Kind Restraints: The team has to tie up Jake when he's accidentally infested, to starve out the Yeerk in his head, and they have to constantly watch him on top of it, because they know the Yeerk will try to morph to free him from the restraints.
Ax's favorite TV show is "These Messages", aka commercials.
Ax and TV in general. But partially subverted in that Ax hates all human music.
Knight of Cerebus: While the series was never particularly lighthearted, the events of the David Trilogy really ushered in the War Is Hell era, forcing the heroes to take extreme measures to attain victory.
Know-Nothing Know-It-All: Rachel's mother Naomi, who doesn't adapt well to going from high powered attorney to camping with aliens and subordinate to her teenage nephew.
Not at first, but Naomi eventually proves herself rather useful, helping the Hork-Bajir draw up their own Constitution and persuading Captain Olston to lend his support to the team.
Know When to Fold 'Em: At the end of the final battle. After it becomes clear that Tom plans to kill Visser One using his own personal Blade ship, the visser essentially surrenders to the kids once they arrive on the bridge.
Similarly, once the Controllers onboard the Pool ship realize what has happened, they surrender to the kids in exchange for amnesty and a chance to acquire the morphing power (to permanently morph animals and move away from parasitism). The surrendered Yeerks got off quite well, all things considered.
In The Capture this is revealed to be a major tenet of Yeerk psychology: Yeerks will give up when the odds don't favor them rather that fight against impossible odds as humans do. This semi-defeatist mindset is presented to explain away the Bond Villain Stupidity of Jake's Yeerk, but later books are consistent with this, as it comes up again in VISSER and The Answer.
Lampshade Hanging: Generally, tropes in this series are lampshaded if they aren't deconstructed or subverted (though a couple are played straight).
La Résistance: Twice, actually. The premise of the series is one, the final arc is the second.
Last Name Basis: Chapman. In the main series, he's the main characters' assistant principal, so it's rather understandable that they refer to him by his last name, with or without a Mr. In The Andalite Chronicles, however, he reveals that his first name is Hedrick, but he prefers to be called Chapman, anyway.
Last Stand: The Animorphs choose fight over flight often, even if it seems hopeless. Maybe especially if it seems hopeless. The fact that humans do this mystifies Visser Three. Visser One recognizes that makes humans very dangerous.
The reader never finds out who burned down Joe Bob Fenestre's house, after Jake told him that inside that house was the only place he'd be safe from them. Jake says that the list of potential suspects includes Visser Three, Cassie, and himself.
I guess you'll never know.
Did Rachel kill David?
In #41, the whole Bad Future thing turns out to be a mind exercise brought on by some other entity that is studying humans. It's not Ellimist or Crayak... So who was it?
The ending of the whole series, actually…more or less. We don't know what the hell The One is, what happened to Ax, or even whether or not Jake, Marco, Tobias, and Ax survived the Bolivian Army Ending.
Les Collaborateurs: Some members of The Sharing. In an alternate timeline where the kids didn't meet Elfangor, Tobias became one, albeit because The Sharing was able to appeal to his lack of friends.
Funnily, not to be confused with their inverted skin tones.
Also, possibly Rachel in The Separation, when she literally splits into two different people after her starfish morph goes wrong. One is timid, caring, gentle, and pacifistic; the other is dark, ruthless, and obsessive.
Lightning Bruiser: Many of the kids use Lightning Bruiser-esque animals as their battle morphs. Jake's tiger and David's lion are the most obvious examples, but Rachel's bear and elephant and Marco's gorilla can also motor when they have to, while remaining very, very, strong. Rachel even notes in the first Megamorphs book that an elephant can outrun a human.
Like Parent, Like Spouse: Tobias's mother Loren is a feisty, fearless, often reckless and smartmouthed blonde who falls for a guy outside her species.... hmm, I wonder who that sounds like.
Ax, and Andalites as whole, do not have mouths and if they morph into a creature with one then the feeling of taste drives them mad. Every time Ax turns human his friends have to stop him from devouring everything from cinnamons buns to cigarette butts.
Yeerks naturally are blind slugs, and possession of their victims is intoxicating because of all the new senses they gain. Even Visser Three, the Yeerk Dragon-in-Chief, fell in love with the sense of sight.
Line in the Sand: Literally. After the Yeerks find out where the Hork-Bajir valley is located, they plan to storm it and kill everyone. Jake and his friends attempt to explain to Toby that going into battle is suicidal, and to demonstrate his point Jake draws a line in the sand and asks the Hork-Bajir to vote on which course of action is the smartest. Jake's point backfires when every Hork-Bajir votes to fight back.
Loads and Loads of Characters: Let's see: Jake, Marco, Cassie, Rachel, Ax, Tobias, David, Tom, Eva, Peter, Loren, Naomi, Jordan, Sara, Chapman, Erek, Visser Three, Visser One, Toby, Jara Hamee, Mr. King... And none of those are one-shot characters, either.
Losing the Team Spirit: The group breaks up for various reasons once or twice. And near the end of the series there's a period where they all fucking hate each other.
Luke, I Am Your Father: Twice. Marco's mother has been taken over by the leader of the invaders. Tobias' father was the war hero alien who gave them their powers. Justified late in the series; the Drode whines that the Ellimist stacked the deck to get Marco and Tobias, along with Ax and Cassie, into the Animorphs.
MacGuffin: Several over the course of the series, including the Pemalite crystal, the blue box, the Anti-Morphing Ray, and the Pemalite ship — but the ultimate MacGuffin is the Time Matrix, which was the catalyst to the Andalites and Yeerks even discovering Earth, let alone the war itself.
Made of Iron: Because the kids can morph or demorph to heal all bodily injuries, it takes a lot to kill them. Typical battles include at least minor injuries. More commonly the kids suffer major injuries; arms being cut off, major blood loss, et cetera. The most memorable example was in the second last book, when Jake, woozy from blood loss, gets shot in the head by a human-Controller. Miraculously he survives long enough to demorph.
Bear-Rachel getting an arm cut off and using it as a weapon also qualifies as memorable.
Also, after reassuming the original form, the morpher's bodily injuries could theoretically be healed. In Megamorphs 2, Tobias says that he should be able to heal his broken wing after morphing and demorphing. Ergo, any bodily injury sustained in the original form of the morpher should be repaired, including brain damage, amputation and the like. Age would remain unaffected as DNA degrades with age: a newborn cloned from a 27-year-old's cells will essentially start with 27-year-old DNA and cells.
James, the leader of the Auxiliary Animorphs, is crippled due to an accident rather than a genetic disease, so after demorphing from pidgeon he finds that he can walk again. Similarly, Loren's blindness is cured by morphing. Marco also heals a dog bite by morphing then demorphing.
A complete list of Jake's nicknames: Big Jake, Fearless Leader, Jake the Mighty, Jake the Yeerk-Killer (sometimes Big Jake the Yeerk-Killer, used mockingly), Jake the Ellimist's Tool, and (in the alternate timeline from Megamorphs #3) Supreme Leader. Ax calls him "Prince Jake" but this is an Andalite military designation.
Mainlining the Monster: The Venber are a sentient race with unusual physical properties, chief among them that if they are brought to a temperature above freezing, they melt. The resulting liquid is apparently an excellent coolant fluid for supercomputers, and the Venber were hunted to the brink of extinction for it.
Mama Bear: Both Loren and Eva qualify, but Naomi, for all her bossiness, thickheadedness and inflexibility, is the epitome of Mama Bear. In one instance, she attacks a grizzly bear that she thinks poses a threat to her daughters Jordan and Sara with a spice rack, and knowing her, probably would've won if it weren't for the fact that the bear was her eldest child.
Manipulative Bastard: Cassie, Marco, David, The Drode. But most of all, Jake, who becomes more and more manipulative as the series progresses.
"[My husband and I] started as ghostwriters, so we saw it more as opportunity. We paid well, but not very well to be honest. We wrote outlines (we suck at outlines) and then got all bitchy when we didn't like what we got. Neither of us is an editor so we weren't really capable of offering decent guidance. So we tended just to sort of slash and burn. Basically without meaning to be we were probably horrible assholes to work with."
Meaningful Name: Sometimes obvious, sometimes not. According to Word of God, Lord of the Rings references are everywhere; Yeerk is a reference to "Yrch", the Elvish word for Orc, and Elfangor's and Aximili's names are references to elvish cities.
According to Word of God, Cassie and Marco are based off of K. A. Applegate and her husband, Michael Grant, respectively. Applegate's first name is Katherine, so she's probably called Kathy a lot, which explains Cassie, and Marco sounds pretty similar to Michael.
Meta Guy: Marco. The others often do it, but Marco makes it an art form.
Mildly Military: The Animorphs themselves. Not at first, but as the war goes on and they get a sense of discipline, they definitely grow into this.
Militaries Are Useless: the military doesn't seem very worried about the Earth being invaded by aliens (see Extra-Strength Masquerade). When they finally admit that aliens are indeed invading, they send some Redshirts to die to support the main cast, and give them heavy weaponry to toy with, but don't have a very important role in the end.
Million to One Chance: At some point the reader may sit back and think, "Hey, wait a second. Yeerks are a race with insanely superior weapons. Not only that, but anyone can be a Controller. And this is a worldwide invasion. The heroes are six teenagers who live in a small town in California that can turn into animals? How can they stop the invasion? A bunch of animals couldn't beat the U.S. Army, never mind the Yeerks." This is lampshaded many, many times throughout the series, as the kids admit that at best all they do is slow down the Yeerks from time to time. They mostly lose battles and they agree that they'll never really be able to beat the Yeerks. They do eventually win, due in large part to the morphing technology being so dangerous and versatile. Rachel sums it up pretty well during David's betrayal when the kids are reflecting on how hard it is to kill an Animorph:
Rachel: Just us. Just us against an enemy that could become any living thing. An enemy that could be anywhere, at any time. An owl in a tree, a spider in your house, a cat in the night, and then... Then, when you were unprepared, when you were vulnerable, a lion or a tiger or a bear. I was starting to see why Visser Three hated us so much.
The Minnesota Fats: Elfangor, but only from Ax's perspective. In fact, the main reason the Animorphs go to rescue Ax is because they feel an obligation to any Andalite because of Elfangor's kindness. From his own perspective, Elfangor is more of a Nice Job Breaking It, Hero.
Mordor: The Yeerks' home planet. Also, in a way, the Yeerk pool.
Mook Promotion: Tom, who becomes an important antagonist late in the series, and a major player in the final battle.
More Than Mind Control: There are voluntary Controllers, people who willingly let a Yeerk infest them. Many of them are simply so alone, so desperate to be part of something, that they're willing to give up their free will. The Sharing's main purpose is to find these sort of people and indoctrinate them.
Mundane Utility: Includes getting keys off the pool floor and getting into concerts for free. Jake tries to forbid this, but he keeps failing miserably... especially when he wanted to go to both concerts.
Mutual Envy: In The Ellimist Chronicles, two Ketran flying crystal cities meet. One Ketran blurts out that the others are building a hydrofoil (which means the crystal requires less manpower to stay airborne) just as his counterpart exclaims that they're building a spacecraft.
My God, What Have I Done?: After the war, Jake is so guilty about ordering to have the Yeerk pool flushed into space and sending Rachel, his cousin, to kill Tom, his brother (the former dies in the process) that he is driven to clinical depression. As Marco puts it: "He wore Rachel and Tom and those seventeen thousand Yeerks around his neck like the Ancient Mariner and his albatross ... He could've snapped his fingers and had anything he wanted, but he didn't want anything. Except for Rachel and Tom to be alive. For Tobias to come back. To unlive that fateful order that doomed seventeen thousand Yeerks."
The whole series exemplifies this, to some extent. The kids agree that self-defense is justified, but the problem is when you kill a Controller, you don't just kill the Yeerk; you're also killing the host, who is completely aware but unable to stop the Yeerk. The kids debate during the entire series what is acceptable when it comes to self-preservation and exactly how far is too far. Initially the kids take a very narrowminded, black-and-white view - "We have the right to do anything we have to to win" - but as they mature and experience more in the war their moral lines become blurred to the point that they don't know the difference between right and wrong.
My Greatest Failure: Jake, whenever he recalls the David incident. Also the fact that Jake couldn't save Tobias from being trapped as a hawk.
Even moreso, the fact that he couldn't save Rachel or Tom from dying, not to mention the order to kill seventeen thousand Yeerks. Those two failures become the entire focus of his personality for about five years.
Mythology Gag: Jake's decision to "ram the Blade Ship" in the series' ending mirrors Elfangor's decision to ram an enemy ship in The Andalite Chronicles. While Elfangor won his battle involving that tactic, the result of Jake's decision is unclear.
“The Abomination” is a name given to Esplin 9466 after he infests the Andalite Alloran. Granted, he fights with a sledgehammer, but do you really want to piss off someone with a sledgehammer of monstrous morphs from dozen of systems?
And from the Yeerks’ perspective, Jake is "Jake the Yeerk-Killer", and he earns the title.
Ax is kind of amused when he learns that Yeerks refer to his brother as "Beast Elfangor."
Narrative Profanity Filter: Being teenagers, and this being a very realistic series, the kids swear and flip each other off a lot. Depending on the "explicitness" of the word, it may or may not be directly written. "Crap", "damn" and "hell" are okay. Nothing else is explicitly mentioned, though the reader can guess the exact word most of the time.
Never Say "Die": Completely averted, and kinda lampshaded when Rachel initially thinks "I'm going to destroy David," but then she corrects herself: she's going to kill him. "Destroy" is a "weasel word" because it's vague and almost meaningless (the exact reason it's considered more family-friendly and often used in kids' fiction), whereas kill means kill.
When a bunch of Star Trek fans insist to help kids protect the Hork-Bajir's valley from a Yeerk onslaught all is splendid until Jake starts giving some last-minute orders, including "to move the wounded to safety but leave the killed lie." The father of the Trekkies family then asks timidly if by "killed" Jake meant "stunned or captured" and Jake replies that no, by "killed" he meant "killed to DEATH".
Never Trust a Title: Some of the later books got really bad about this. Titles like The Suspicion (where nothing is suspicious), The Prophecy (which features no prophecy), and The Hidden (which features a bizarre morphing buffalo that is definitely not hidden) come to mind.
Strangely, these are all Cassie books. Make of that what you will.
That and Seerow being responsible for the Yeerks becoming a danger in the first place.
Jake's handling of David in The Threat also counts.
No Biochemical Barriers: Hork-Bajir can eat Earth bark and Andalites can eat Earth grass. Yeerks can interface with all sorts of alien nervous systems, suggesting a common signaling system. Also, it seems DNA is very common, forming the primary building block of almost all living organisms around the galaxy.
Noble Savage: The Hork-Bajir before their war with the Yeerks.
And yet, real celebrities' names were dropped all the time. Noah Wyle of ER was mentioned a lot, and book 17 gives us no less than seven real celebrity names at the Planet Hollywood concert. Cassie and Jake even use David Letterman as a code word when talking about David in an insecure phone conversation.
They also meet a world leader at the summit in the David trilogy who is almost certainly Boris Yeltsin.
In #23, Tobias' last name is transcribed as "______" in dialogue. One fan theory is that his last name is Fangor, since Elfangor was his father and used the human name "Alan Fangor." Another fan theory is that, because The Ellimist erased all trace of Elfangor from Tobias' mother's memory and she married someone else, even though Tobias still existed somehow, Tobias was given that man's last name. Whoever he was.
The revelation of Jake's last name (Berenson, which is probably also Rachel's) is used to highlight the seriousness of the new situation.
No Paper Future: Used to comedic effect and to demonstrate Applegate's mild dislike of computers: Ax can't believe that books were invented before computers, because he finds them much quicker and easier to use. (He's also surprised that the telephone was invented before the chat room.)
No Sidepaths, No Exploration, No Freedom: Alternamorphs #1 is a rare literary example. Despite ostensibly being a Choose Your Own Adventure type gamebook, the author doesn't seem to understand the concept, because the story is completely linear and every "wrong" choice results in instant death. Oddly enough, the second book went to the opposite extreme to the point of unintentional deconstruction.
Nostalgia Filter: An in-universe example: at the end of the series, Marco sees the years he spent fighting Yeerks as the "good old days". He remembers life-and-death battles as "cool, rock 'em sock 'em battles". He doesn't really seem to remember how much they scared the crap out of him at the time. But then, it's said that Marco has a much easier time adjusting to civilian life than the others, because he doesn't feel guilty about the things he's done.
Nothing Is the Same Anymore: After Visser Three's promotion to Visser One, the war quickly escalates and missions become increasingly important. This culminates in the discovery of the "Andalite bandits'" true identities. The seriousness of the kids' new situation is highlighted by the revelation of Jake's last name.
Not Quite Dead: After David leaves the barn, Jake sends Tobias to follow him. When Jake catches up, David says that he's killed Tobias, and Jake sees Tobias' mangled corpse. But as it turns out, that wasn't really Tobias, just an innocent red-tailed hawk that happened by. David had simply lost Tobias early in the evening, and the latter had spend a good couple hours trying to find him.
While the Helmacrons and the kids are inside Marco's body, he morphs into a cockroach. The Helmacrons shoot Marco's heart, rendering him ostensibly dead. But, as Cassie suddenly recalls, stopping a cockroach's heart doesn't kill it - they have a backup system.
Spoken word-for-word near the end of the final battle:
Visser One: So. Still not dead. Jake: No, visser, not quite dead.
Not So Different: There's some angsty speculation among the kids that taming and controlling their morphs' natural minds is the same as what the Yeerks do. Likewise the whole-sale slaughter of Yeerks, and the possible comparisons to terrorist attacks on civilians.
Cassie gets an entire book dedicated to this in #19: The Departure
Crayak also does this with Rachel.
Also done with the Howlers and the Pemalites, two alien species created by Crayak and the Ellimist. The Howlers look like Always Chaotic Evil who kill everything in their path and the Pemalites are insanely pacifistic, but the two races, thanks to how they were designed, actually had the exact same priority in life: to have fun. Their creators just gave them different ideas of 'fun'.
Not So Extinct: The Venber in "The Extreme". Though they're actually hybrids of Venber and humans brought back by the Yeerks.
Number Two: Marco fits the traditional role of The Lancer, but Tobias seems to fit this role. Jake usually appoints him leader of the Marco-Tobias-Ax sub-team, which ended up carrying the end of the war.
Obviously Evil: Averted, as the Yeerk in a Controller's head is indistinguishable from the Controller, as the Yeerk just knows the thoughts of the host and acts accordingly. The one exception is Visser Three, who just oozes evil (being the only Andalite-Controller in Yeerk history, he doesn't need subtlety).
Off with His Head!: Visser Three's preferred method of execution, though he's a fan of torture as well.
Oh Crap: A couple times, but most notably in the second-last and last books. First, when Visser One realizes that the Animorphs have gotten onboard the ship, and second, when Tom discovers he's been had and the kids are still alive.
One-Winged Angel: Technically all of the "battle forms" count, though Visser Three gets to do a classic example at least once every few books.
Online Alias: Mostly features the "modern" kind: YeerKiller9, Gump8293, Bball24, etc. But it also has some more hacker-like ones, like "Govikes" and "YrkH8er". The Ketrans use gaming names as well. Like "Ellimist".
Open Says Me: In The Warning Jake morphs a rhinoceros to invade Joe Bob Fenestre's home. The other Animorphs have to guide him due to the rhino's poor eyesight, and frequently have him 'open doors' where none existed before.
Opt Out: Right before the final battle, a lot of the Auxiliary Animorphs decide that they want to sit it out, due to the fact that their friend Ray was recently killed. Jake doesn't really care and makes them go anyway. ("We didn't give them morphing power so they could have fun flying around. This is when we need them. All of them. You're their leader, James, so lead.") They then all get killed, which Jake expected to happen.
Our Centaurs Are Different: Andalites are blue centaurs with scorpion tails and stalk eyes. In one book, a TV show gets a few seconds of an Andalite on film and wonders if it proof that centaurs exist.
Out-Gambitted: Visser One's plan worked well. Tom's worked even better. Jake's worked best of all.
Overly Long Name: The Ellimist's real name is Azure Level, Seven Spar, Extension Two, Down-Messenger Forty-One. Ketrans' names are basically their address, which is why they tend to call each other by their chosen names or gaming handles, and his was Ellimist.
Painful Transformation: Actually, it's specifically mentioned that while there's no actual pain, it definitely feels like there should be.
Panthera Awesome: Big cats make good battle morphs. Jake's main morph is a tiger, while David and James both had lion morphs. There were also times when the whole group morphed jaguars and cheetahs. Hell, even Tom's favourite battle morph is the jaguar.
Parent with New Paramour: Marco's dad eventually marries Nora Robbinette, his son's math teacher.It doesn't last. Eventually, he's reunited with his Not Quite Dead first wife, and Nora becomes a Controller. Marco, either because it'll be easier on his father or because he wants to see his parents together again, lets him think that Nora was always a Controller. God only knows what happened to the dog...
Parental Abandonment: Tobias' dad is dead (his dad is Elfangor, by the way), his mother is blind and an amnesiac, and therefore unable to take care of him. Tobias is "raised" by his aunt and uncle, neither of whom want anything to do with him.
Pay Evil unto Evil: Jake, before flushing the Pool into space, decides that Yeerks are subhuman parasites who deserve nothing but cold, frozen death: "They could've stayed home, I thought. No one had asked them to come to Earth. No more than they deserved. Aliens. Parasites. Subhuman."
Also, Marco isn't very good at hiding the fact that he takes pleasure from killing Yeerks. In #19, he tells Cassie, "You don't make peace with parasites. You don't turn them around. You bury them."
It eventually subsides, though, as part of Marco's Character Development; over the course of the series he becomes much less emotional, which makes him a more effective strategist.
Percussive Prevention: When Ax realizes that the only way to avert a world war in book #46 is to threaten the Yeerk pool, and everything above and around it, with nuclear destruction, Jake, naturally, objects. Ax knows he can't force Jake to have any responsibility for it, so he calmly knocks Jake out.
Perfect Pacifist People: The Pemalites, a ridiculously peaceful race who were completely obliterated by a more militaristic species. Tragically, their incredible technology could have been converted into weapons that would easily destroy their opponent, but as a species they couldn't bear to do so.
There's also the Hork-Bajir, who while not as advanced, were also totally peaceful, and were enslaved when they caught the attention of a more aggressive race. Notable in that both of these species were created artificially, and designed to be non-violent by their creators. The series seems to give the impression that while being a pacifist sounds great, in reality it isn't a good survival tactic.
Phlebotinum Breakdown: Happened a couple of times with Body Horror-riffic results. Rachel suffered a case of Involuntary Shapeshifting, turning into crocodiles, ants, and elephants at inconvenient times. Marco got it even worse, turning into a series of Two Beings, One Body creatures. (Dude. Osprey-Lobster. Trout-Gorilla. Neither of which could breathe. And, of course, the mighty poo-bear! [Poodle-Polar Bear.])
Playful Dolphin: As you'd expect, the dolphin morph is pretty playful. Marco and Cassie force Jake to morph dolphin in an attempt to counter his depression with the dolphin's natural chirpiness. Also subverted, as the morph comes with strong Blood Knight tendencies.
Poor Communication Kills: The Ellimist's race loved to play simulation games based on manipulating populations into developing along certain lines and using them to compete against other players. When another race intercepted a few of these being broadcast they thought they had discovered an utterly evil race of conquerors with the blood of trillions on their hands and wiped the entire race out in a preemptive strike.
Powder Trail: The villain uses this to blow a hole in HMS Victory in Elfangor's Secret.
Power at a Price: To quote Jake: "The power made us responsible, see. Without the power the knowledge would have just been a worm of fear eating up our insides. Bad enough. But it was the power that turned fear into obligation, that laid the weight on our unready shoulders ... Power enough to win? No. Power enough to fight? Ah, yes. Just enough, little Jake, here is just enough power to imprison you in a cage of duty, to make you fight..."
Power Nullifier: The anti-morphing ray. Though we never actually find out if it works —- the Animorphs just manage to convince the Yeerks that it doesn't.
But Marco and Jake's friendship is a theme in the series. Jake and Marco have been best friends since they were old enough to talk, and are immensely loyal to each other. It's said that Marco only initially joined the war effort because of his loyalty to Jake, and throughout the series that loyalty is the source of many crowning moments of heartwarming. Marco is the only one who never criticizes Jake when it's apparent that he's already under a lot of stress, notable because normally 99% of criticisms come from Marco.
Made even more heartwarming by the fact that Jake is the only person Marco really respects. Everyone else just brings out Marco's sarcastic Deadpan Snarker mockery.
Power Trio: Marco, Ax, and Tobias were often sent alone on missions, especially near the end of the war. Ax and Marco switched off, but Tobias was always Ego/Kirk.
Power Parasite: Following this trope literally, Yeerks use other creatures as hosts for their specific abilities.
Pretty Boy: Ax, in his androgynous male human morph, which is made of DNA from Rachel, Cassie, Jake, and Marco.
Properly Paranoid: All the kids to some extent, but Marco takes it really really far, mostly because he's the most afraid of dying.
Marco: I'm paranoid, sure. But that doesn't mean I don't have enemies.
Proud Warrior Race Guy: Ax, though it's explicitly said that Andalites aren't really a warrior race. Andalites are supposed to love peace, and spending time with their families, and communing with nature, and so forth. But Andalite warriors, while pretending to love those things, are really only interested in fighting for the glory of the Andalite people.
Andalites are not actually psychic. Thought-speech seems to be a physically measurable phenomenon (the Nisk had thought-speak detectors; Andalites have thought-speak microphones) present in the brains of some species that allows them to broadcast thoughts. Even more supported in that only linguistic information can be transmitted, suggesting that the transmitter is located in the language centers of their brains. Apparently absolutely any brain can act as a receiver without any specialized neural structures.
Further supported while simultaneously providing an explanation for why the morphing technology grants thought-speech to those in morph. If an Andalite's thought-speech transmitter was a physical structure in their brains, they would lose it during a morph and thus be unable to thought-speak unless the morphing technology included that feature.
Psychic Static: John Berryman, Jr. thinks the play Henry V at Visser Four, so much so that the very first thing he does when he finds the Time Matrix is try to change the result of the Battle of Agincourt so that Shakespeare would never be inspired to write it.
Punished for Sympathy: The Yeerks consider sympathizing with or befriending the host the worst possible crime, and their laws punish it with a painful death sentence.
Punny Name: "Animorphs", a portmanteau of Animal Morphers, was a name originally devised as a joke by Marco, reminiscent of the Justice League and other superhero team names. The kids more or less jokingly adopt the name, but refer to themselves as "the Resistance" or something similar to outsiders. By the end of the war, their enemies begin seriously calling them Animorphs, and they begin calling themselves Animorphs seriously as well.
The Purge: Late in the series, a massive campaign to reorganize the Yeerk Empire begins, headed by Visser Three. Visser Three convinces the council that Visser One is a traitor, and she is executed. Visser Three usurps her position and kills everyone loyal to her, replacing them with his own subordinates.
Put Off Their Food: In The Android, Marco's parents try to serve him chicken for dinner after he was nearly killed by a bird while in spider morph. Marco opts out.
Pyrrhic Victory: In book #47: The Resistance; and in the final battle, in books #53-54.