The Autobot commander of the forces on Earth, Prime leads the team through the Mini-Con Battles, against Unicron (twice) and finally in an effort to save Cybertron and reality from the Unicron Singularity.
A Father to His Men: Goes absolutely berserk after Smokescreen's near death. It's seen quite often throughout the show, which makes him a particular contrast to Megatron throughout the trilogy.
Anti-Hero: While he's still, without question, a hero, it's worth noting that this Prime seems more willing to resort to violence compared to others. This is exploited by Sideways/Unicron who accuses Optimus of being as much of a Blood Knight as Galvatron, despite Optimus claiming he fights merely because he has to. Unicron is proven somewhat right in the Armada finale when Optimus and Galvatron have their final brawl in that series.
Look What I Can Do Now: In Armada, he powerlinks with Jetfire, and later the pair link with Overload. Normally this equates into instant win. In Energon, he powerlinks with more or less the entire Autobot army, forming Optimus Supreme to fight a revived and very, very pissed off Unicron.
In Cybertron, he combines with Leobraker to form "Savage Claw" mode and Wing Saber to form "Wing Optimus".
Smokescreen originally intended to be one of the team members who accompanied Optimus Prime to Earth, but he entered the spacebridge portal late, and as a result, he materialized some time later then they did.
Badass: If being lethally wounded by the Requiem Blaster and still having the strength to try and take Megatron down doesn't count, what does?
Demonization: The kids all assumed that he was a Decepticon when he first appeared. Of course the fact that he chased them all over the place and didn't introduce himself properly probably didn't help much.
Hot Shot's newest partner, it didn't take long for Sideswipe to be captured by Hot Shot's friend-turned-foe, Wheeljack. Sideswipe was saved but the experience taught him a harsh lesson about war, leading him to quickly sober up.
Leader of the Decepticons. Megatron is a malicious, powerful leader. A refined, focused general who treats his men with little care, yet still infuses them with genuine respect for him and the Decepticon cause.
Affably Evil: Armada. When it comes to the next two series, however...
Badass: From the middle of Armada, and all the way through Cybertron.
Adaptational Badass: No Starscream since then has come close to doing what he's done in Cybertron. Fights Primus twice and lives? Check, though he still loses both times. Only non-comic relief Decepticon left alive at the end of Cybertron? Check. Fought off eleven Autobots to steal the Omega Lock? Check. While his living up to the trope he named has been done by otherStarscreams, he's done the best to date.
Big Bad: In Cybertron. Could be seen as a secondary one in earlier Armada.
Determinator: In all series of the Unicron Trilogy he has his moments. The Cybertron incarnation in particular is always trying to get ahead through schemes, and just never gives up. Even at his lowest point after his battle with Landmine, he pulls himself back up and displays such power equivalent to Galvatron's, the Autobots are defeated by his shock waves. His speech to Galvatron basically boils down to he's risked everything for power, and he's not going to yield.
Heroic Sacrifice: Twice. The first happens when he keeps shooting at Unicron in an effort to convince Galvatron that both sides must ally. The second occurs at the end of Energon, when he follows his master into the giant sun.
Noble Demon: In Armada and in Energon. Not so much in Cybertron.
Dumb Is Good: He's the closest thing to a "nice" Decepticon that you're going to find.
Only Sane Man: Compared to the rest of the Decepticons, he definitely feels this way.
Pet the Dog: A quite memorable one in Armada; following a defeat, Megatron decides to pass his nerves on Starscream by taking him to a room and delivering him a No-Holds-Barred Beatdown. Demolisher, realizing Megatron might well kill Starscream, breaks through the door to take his defense.
Tank Goodness: In Armada. In Energon, he becomes a dump truck instead, but reverts back into a tank in Cybertron.
Undying Loyalty: Played mostly straight to Megatron, but the writers do subvert it at the end of Energon. When Galvatron hurls himself into the sun to stop Unicron and Starscream and Mirage follow, he and Snowcat don't bother to do the same.
Spared by the Adaptation: The Energon comic shows him as in charge of Cybertron's security systems and one of only two Cons to not immediately defect at the start. It probably helps that in that continuity, he didn't defect to Unicron and is much more loyal.
Stupid Evil: He overestimates himself far too much when he defects.
The Strategist: He's actually pretty good at strategy, just not very good at improvising.
Undying Loyalty: To Megatron. Which may come across as oddly amusing, considering his VA voiced the treacherous (and much higher pitched) Terrorsaur
Wheeljack, formerly an Autobot serving under Hot Shot. When Wheeljack was pinned beneath some debris during a fire, Hot Shot was forced to leave him to get help, but was prevented by his superiors from returning. Wheeljack believed that Hot Shot had abandoned him, and when Megatron found and rescued him, he swore loyalty to the Decepticon commander.
A primal force of evil, his origins lost to the mists of time. He is the embodiment of all darkness and hate that lurks in the hearts of all beings.
Voiced by: Mark Acheson, David Kaye (when Megatron is possessed in Energon)
Back from the Dead/ Death Is Cheap: This happens an incredibly frequent amount of times. You honestly wonder why the bots try to put him down again when he just gets back up.
The first is implied at the end of Armada, when his planet form is seen floating behind an unknown body. It's later confirmed at the start of Energon that he's just mentally inactive.
The second and third attempts happen during Energon. He manages to reactivate his body form and possess Megatron, then dies a few episodes later at the hands of Optimus Supreme. Then after that he possesses Megatron yet again, turning him into the incredibly large Galvatron in an attempt to recreate his body. This is thwarted when Galvatron hurls himself headfirst into a sun.
A fourth and seemingly definitive example from the Cybertron comic: Unicron frees himself from the Black Hole by hijacking a smaller Transformer's body, then recreates his body some time later. This time, he flees before Primus and the Autobots can defeat his weakened body.
Planet Eater: Though unlike his debut appearance, he doesn't physically eat them, rather breaking the planet into smaller pieces and then devouring them. Energon shows this best.
Sci-Fi Writers Have No Sense of Scale: Not as bad as his first appearance due to the fact that he rarely engages any Cybertronians physically, but when Thrust is perched/standing on his shoulder you do run into some fairly notable issues, notably the fact that he should theoretically be a speck from the distance we see him.
An intergalactic drifter, Sideways transforms into a motorcycle. His consciousness is apparently housed within the smaller robot head that rides his alternate mode, which can split into two components.
Technically, he first appeared in Robots in Disguise with a toy from that show's line, though he was just an Autobot warrior.
The Dark Side Will Make You Forget: Servitude to Unicron caused him to forget that he was hoping to avenge his planet, and turned him into the Jerkass we see him as in Armada. Subverted in Cybertron where he remembers again.
Expy: Averted with his other appearances- possibly. It's thought to be the same guy throughout the multiverse.
Even Evil Has Loved Ones: It is implied that Unicron was in possession of Sideways during Armada, thus temporarily draining any sense of compassion the latter may have had. In Cybertron, with Unicron destroyed, it is shown that Sideways does has the ability to care; in this case for the inhabitants of his destroyed world, including Soundwave and Laserbeak.
The Heavy: After he appeared, he gradually shifted towards this, particularly as his boss was not shown for most of the series. Even when his boss appears, he serves as the representative and does most of the fighting.
Kick the Dog: He enjoys doing this. Particularly when it simultaneously helps his goals and hurts others.
Manipulative Bastard: He can quite easily turn people into his puppets when he needs to (not literally). Even in Cybertron, he has a tendency to do this, as he manipulates both sides again, taking advantage of their amnesia as a result of the Unicron Singularity.
Mind Screw: Unicron, Sideways, Sideways' Mini-Cons. It's kind of hard to tell where they each end and the others begin, which isn't helped by the shaky dub. Of course, he enjoys running on this.
The Mole: To Unicron, betraying the Autobots and then the Decepticons in the process.
Mouth of Sauron: His boss Unicron can talk, but to an extent Sideways still serves as this.
Playing Both Sides: He initially screws over the Autobots by pretending to be a Decepticon, then in turn he screws the 'Cons over by leaking information of theirs to Unicron.
Troll: He enjoys screwing people over, particularly when it helps him advance his plans.
Villain Decay: In Cybertron; He started out as a shadowy manipulator, playing both sides through sheer cunning. He became something of a joke in the later episodes, being outwitted and made a fool of by the humans (Though he gets better during his last few appearances). He even lampshades it at one point:
"I'm the one who's supposed to be pulling everyone's strings!"