Raised by Wolves: Natsu seems capable of intelligence. He probably could stand next to Lucy as a rare island of sanity in Fairy Tail if he'd had human parents instead of a dragon dad.
Random Power Ranking: Erza is essentially as strong as she needs to be, powerful enough to match up against some of the Ten Wizard Saints and probably actually stronger than at least two. She's also not the most powerful of the S-Class wizards in the guild. Erza and Makarov have also noted that the presumably A-Class ranked Natsu is on par with Erza when he gets angry. Which... isn't really true when you've seen the two fighting their opponents, but still throws the whole ranking system into question.
Both justified. Being one of the Wizard Saints is a title granted by the Magic Council, which is a political body, so obviously they must be perfectly impartial. The S-Class rank is certified individually by each guild and each may have their own standards. In Fairy Tail S-rank is granted by a test and involves more than just fighting ability. While Natsu is Erza's equal in combat, he's far more impulsive and generally much worse at handling any situation that can't be solved by beating someone up than she is.
Really Gets Around: Gildarts names twelve women off the top of his head that he's slept with, then two more when trying to guess which one he fathered Cana with.
Red Baron: Erza the "Titania/Queen of the Fairies", "Demon" Mirajane, "Beast Arm" Elfman, "Salamander" Natsu, "Sky Sorceress" Wendy, "Black Steel" Gajeel, Juvia of the Deep, and "Darkness" Fried. And these examples are only from Fairy Tail itself.
Laxus with Fried, his dragon. Also with Mystogan, his rival.
Reformed, but Rejected: Hoteye and Jellal. Sure, they had a change of heart, but the law still holds them responsible for their past actions.
Same goes for Zeref. Though, in fairness, given his reputation, it's hard to blame others for still shitting bricks just when his name is mentioned. However, unlike the aforementioned examples, he eventually decides that Humans Are Bastards because of it, and that if they won't accept him, so be it.
Destroy the Lacrima in each of Nirvana's six legs to stop it? OK, but it had eight legs the first time we saw it. In the manga, at least it had seven legs at first not eight. In then repeatedly changes between eight and seven. Chalk it up to lazy author/assistant not counting right.
So Lisanna is alive. She was in Edolas the whole time? It was implied that she was killed by Elfman's Demon Form.
When Gajeel and Juvia joined Fairy Tail it was said that they were instantly promoted to S-Rank wizards. As of Chapter 201 they apparently aren't, but that one can actually be explained: They were considered S-Class before joining, but have to retake the test to be S-Class in Fairy Tail.
Natsu's motion sickness is extended to be a universal trait of Dragon Slayers. Even acknowledged as a retcon; Gajeel was actually surprised when this one kicked in.
The Reveal: The hooded person that appeared through the Tournament Arc and had a magic similar to Zeref's is revealed to be Lucy from the future.
Revealing Hug: Jellal does one after Erza "defeats" him and "removes Zeref's influence". Obviously he wasn't in control, and used the hug to place a binding spell on her
Reverse Mole: Gajeel, Ivan's mole in Fairy Tail, was actually a double agent.
Whenever Lucy comes home, she is greeted by one (or more) of her comrades nonchalantly sitting on her couch having broken into her home. Every time. Without fail. Usually Natsu, with Gray and/or Erza accompanying him from time to time.note It's revealed in an omake that Mirajane gives them the spare key.
Every time Team Natsu comes back to Fairy Tail, eventually it will end up in a Bar Brawl. Always.
Max ending up with his pants down and with a broom up his butt.
Erza taking everything way too seriously.
Natsu scaring the crap out of a monster and is called one.
Natsu: "I'm not a monster."
Lucy walking home from the guild on top of the canal levee, while someone on a boat in the canal warns her about falling in.
Natsu getting Lucy's name wrong.
Gray making Mirajane cry.
Virgo asking for a punishment for no reason.
Wendy and her intense dislike of Umeboshi (pickled plums).
Sadistic Choice: Laxus imposes one on the Fairy Tail members: To get out of his magic traps and rescue their True Companions whom he had taken hostage, they must fight each other.
It happens again when Jackal of Tartarus forces Lucy to choose between a pregnant woman and the ex-council member that Fairy Tail was trying to protect. Fortunately Natsu arrives with a Take the Third Option and attacks Jackal instead.
Schizo Tech: The world is largely a medieval to Renaissance era equivalent, with the most sophisticated conventional piece of machinery being the steam train. Anything more advanced utilizes magic as a power source (temporarily draining the magic energy from an user via a special plug) and is both rare and powerful. The gamut of magical devices ranges from firearms and automobiles, to airships and computers, to Humongous Mecha and a Kill Sat. Rock Beats Laser is very much in effect, though, as magical devices harness a fraction of one's true magical potential and thus can be surpassed by those who wield magic directly.
Senseless Sacrifice: Subverted when Ultear sacrifices herself to rewind time. She only manages to get back one minute and dies thinking it was all in vain, but it was just enough to prevent the death of at least half a dozen characters.
Sexy Mentor: Gray's mentor, Ur. She was the one to give him his habit of stripping after all.
Shameful Strip: Yukino's master forces her to remove her clothes and guild tattoo in front of the entire Sabertooth Guild after she loses one fight. Then he kicks her out.
Shapeshifting Excludes Clothing: At one point during Team Natsu's fight with Hades, the latter used a technique named "Katsu" on Wendy, causing her to disappear instantly, only leaving her clothes behind. Fortunately, it turns out she was saved at the last minute by Horologium, and that Hades' attack actually didn't hit her.
Happy is a major offender here, but his favorite couple seems to be Lucy×Loke. It's a bit of a stretch to say that Happy actually ships them though. Its more like he's making fun and teasing Lucy because Loke couldn't have made it more obvious that he wants Lucy. Some argue that Loke wants all girls.
Chapter 281 has him pretty big on Jellal/Erza too. He clearly saw their private moment at the beginning of the arc.
In the Nirvana arc, Virgo comments about Natsu having a crush on Lucy, but she could just be imitating Happy.
Lucy: "Even if it was true, I wouldn't go for it."
Ship Tease: Used so much that a case can be made for practically any character falling for any other one. To try to figure out who is supposed to be an Official Couple is to submit yourself to madness. But Jellal×Erza is heavily implied.
Lucy falling to her death yet having absolute faith in Natsu, and Natsu appearing right on time catch her, and Lucy was very, VERY happy to see him. Natsu×Lucy teasing check.
The special Chapter/Episode 50 "Watch Out For That Guy" is full of Natsu×Lucy ship tease.
Heavily injured Natsu carrying Erza on his arms, with him walking like a badass and her crying on his chest and add a Headbutt of Love. Natsu×Erza teasing check.
Any arc closure event with Erza and Jellal next to each other. Jellal×Erza teasing check.
The very fact that Lisanna came back; Natsu×Lisanna teasing check.
Chapters 207-210 provide a case for both Elfman×Evergreen and Levy×Gajeel. In all honesty, though, that last one was kind of expected.
When Levy leaves "Iron" for Gajeel to eat... the "o" has a heart in the middle. Just saying, not subtle.
Guess which letter Gajeel eats first?
Any time Juvia and Gray are together has Gray×Juvia teasing, if only on Juvia's part. Though he didn't seem unhappy about dressing up as a king to her queen for the Fantasia parade. And Gray gets pissed when Lyon tries to ask her out.
Chapter 366 has Gray finally showing some affection towards her, by covering her hand with his own. But considering whathasbeengoingon during that arc, it can very easily be considered justified.
There's also some Gray×Lucy teasing every now and then, from Lucy being impressed by his scar to her crying to him during Erza's dream funeral. During the Nirvana arc, Gemini (copying Gray's appearance, thoughts, and abilities) reveals what Gray thinks of Lucy: "Newcomer to the guild, looks are pretty much to his liking, somewhat interested in her, more naive than she appears, and a Celestial Spirit Wizard."
Macao and Cana had a bit of Ship Tease in the Phantom Lord arc, which is helped by Cana having been stated to have once had a crush on Macao. This was expanded on in the anime's depiction of the arc, featuring among other things Macao Taking the Bullet for Cana and Cana crying to him when the guild is destroyed.
A lot of the time when Lucy and Loke are together. Especially when they are saving each other. He even says "I love Lucy" at one point.
Lucy×Loke teasing are strongest when they first meet, where she actually blushes and stammers to his advances. Afterwards though, its mostly one-sided on Loke's part. May have to do with the fact that she is used to his playboy attitude and may not be taking his confessions and advances seriously anymore.
Erza and Jellal gets this the most by far. Their relationship is so close though, that many would argue that those moments are far beyond just simply teasing.
Natsu and Lucy come in 2nd place, though it is mostly on the part of Lucy since Natsu is an Idiot Hero who seems to be Oblivious to Love. Lucy does seem to be the girl Natsu is most affectionate towards, especially as shown in the extras. Their teasings are everywhere, just not anywhere as blatant as Erza×Jellal. Juvia and Gray would be 2nd except for the fact that it is made blatantly obvious that Juvia is into Gray, which may not make her actions count as teasing.
Word of God has stated though, that he does plan on making them canon, it's just a matter of how to get there.
Gray used to follow Erza everywhere when they were little, and he eventually promised that he would make sure she never cried again. Gray×Erza teasing check.
Ultear implies that Jellal's willingness to risk exposing himself in public has less to do with wanting to help Fairy Tail win the tournament and more to do with showing off in front of Erza.
Freed x Laxus. Freed has recently been confirmed at least as bi (if not gay) in the extra chapter of Volume 33, in which he is shown reading the 3D Sorcerer magazine featuring nude Blue Pegasus members. Unlike Macao and Wakaba, he's particularly interested in seeing the Blue Pegasus Trimens posing nude. Guess whose picture is hanging on the wall? Laxus.
Episode 45 gives a very small bit of Ship Tease for Freed X Mirajane, from the heartfelt speech reminding Freed that he has friends to Mirajane saying "How about next year, we can just enjoy the Harvest Festival together?" while blushing.
There has been some tease regarding Evergreen×Elfman since the S-Class Trial. In Chapter 290, it turns out that Evergreen was by Elfman's side the entire time he was recovering from his battle with Bacchus. This leads to Elfman referring to Evergreen as a real WOMAN. Not OTOKO, he said ONNA (for those who get the Japanese). Hiro-sensei, we all know what's gonna happen, so make it happen already!
There's also some Leo×Aries, since Leo pretty much defied his owner, indirectly got her killed and spent 3 years in Earthland because he didn't want Aries to be abused by her owner in such way. Also, Eternal Love for extra cuteness.
The Tartarus Arc gives some for Mirajane X Laxus, as she sits by his hospital beside, just like Elfman does for Evergreen.
Omakes lately seem to be even shipper, if that is even possible
The omake "413" was mainly Gruiva, but it had scenes just meant to excite the fanbase for NaLu and GeLe. One scene was Natsu and Lucy sitting next very close to another, with Natsu's arm around her. Lucy saying not in public. They even had a scene of Gajeel and Levi kissing. Of course both of these scenes were Juvia imagining it.
The Natsu and Asuka omake had Alzack and Bisca's kid Asuka try to make Natsu and Lucy kiss. Natsu was happy to oblige, Lucy was blushing like crazy. They didn't though. Later in the story, Lucy realizes that Natsu let Asuka win their target-shooting contest(going by his accuracy in helping Asuka shoot down the flying bandits)...she gives him an adoring look while thinking "He definitely has a good side, huh..."
This gets a sort of call-back in the Grand Magic Games. Gray unleashes his ultimate Ice Make technique. It's called Ice Make:Unlimited.
The outfit Erza dons during the Fantasia Parade in the Fighting Festival arc in the anime and in an extended chapter is an outfit Resha is wearing on occasion during Rave Master.
After knocking out Erza, Natsu turns and gives Jellal a pissed off look, letting him know he's going down. Haru has this exact same expression with the exact same background when threatening Sieg in Rave Master.
It just so happens that Haru gave Sieg Hart that same face and threat just after setting down an unconscious Elie, and that in both manga series, the guy with the blue hair and tattoo across the right half of his face ended up using a spell with the same name which would have killed the hero, had the girl not regained consciousness and tried to save him (though a third person got involved in Fairy Tail), and that both arcs begin with the girl having a winning streak in a casino and end with fireworks.
This one is anime-exclusive. When infiltrating the duke's manor during their very first job, Natsu dresses up as a ninja, degozaru.
Fried's latest glyph encases him in metallic armor with an expressionless faceplate, and it's called Zetsui.
The third and final set of dolls that Bickslow uses in his fight against Lucy are identical to the Black Airs from Mashima's 2006 manga Monster Soul.
Happy's parents are voiced by Tomokazu Seki and Ayako Kawasumi, the same people who voiced Haru and Elie. This is most likely a shout out to Rave Master but it could also just be a coincidence since those two often voiced characters who are romantically linked.
In Chapter 247, Lily steals a sword from a mook. Said mook mentions that his Musica sword cost a lot.
The mook also looks identical to Bis, who was the main mook of the Rave Master arc starring Musica.
When getting the backstory on Ultear's spell, "Last Ages", we find out that she learned it through a combination of books and the ancient Mildean language.
One of Grey's spells is called "Freeze Lancer". It's identical in every way to a recurring arte from the Tales Series.
In Episode 90, the heroes are trying to stop what amounts to a Colony Drop by pushing it back. The green light that shrouds the island when the Exceeds show up, as well as a shot of Exceeds getting thrown away (and being caught by their comrades) make the whole thing look like the Axis drop from the end of Char's Counterattack.
In one of Loke's attempts to woo Lucy he shines a bit of light on the wall spelling out I love Lucy
During the S-Class Trial arc, this exchange occurs in the dub.
Erza Scarlet has a Shower of Angst after retreating from the battle with Phantom Lord.
Lucy has one just before the beginning of her Bathtub Bonding Scene with Cana.
Lucy has a Shower of Angst after losing her fight against Flare Corona in the Grand Magic Games.
Sit On The Camera: in "The Truly Evil Jiggle Butt Gang" the gang does this a number of times.
Slapstick Knows No Gender: It says a lot that Lucy is the show's biggest Butt Monkey, as Team Natsu can attest to. To be fair, Erza asks someone to hit her when she thinks she screwed up. They're all just too afraid to do it. It's also goes both ways with Erza causing the most damage.
The next one turned out to be Gray's Rival Turned Evil who wasn't really any stronger than him.
Then he was followed by a foe of Master Makarov himself that was hellbent on destroying Fairy Tail.
After that came Jellal, a member of the same organization Makarov belonged to and arguably the strongest Big Bad so far, whose goal was nothing less than the resurrection of the Ultimate Evil of the setting, the Black Wizard Zeref and only lost because of the setting's equivalent of SuperSaiyan, a conveniently-timed injury acting up, and the fact that Zeref was never dead, and therefore could never have been resurrected in the first place.
The one that followed (Laxus), was pretty powerful but wasn't a serious threat due to him actually caring for Fairy Tail.
The Algorithm may have returned in the Oración Seis arc with the Big BadZero, Brain's Ax-CrazyOmnicidal ManiacSuperpowered Evil Side, wanting to use Nirvana's power to destroy everything and who managed to one-shot Jellal, though it could be argued that Jellal is still the stronger considering that he had amnesia at the time and had taken a rather severe beating beforehand.
The Edolas arc originally had the Exceed Queen, who is described as being a god, but it turns out she was never a threat and the real Big Bad is King Faust, who's only power is operating a Humongous Mecha.
The S-Class Trial arc has many things coming together at once, since recurring villains are finally putting their master plans into motion (like reviving Zeref, the Big Bad and Ultimate Evil of the entire series). Even the Quirky Miniboss Squad has members quite a lot stronger than previous arcs' Big Bads, and Elite Mooks nearly defeated Gajeel and Levy.
The arc's Big Bad, Hades, destroyed Makarov without even trying.
The Grand Magic Games arc has Arcadios. While he has yet to be shown to have any magic powers, he's in charge of a Rune Knight squadron and has the King of Fiore's ear, making him one of the most powerful enemies yet in an entirely different sense.
Subverted. The arc's big bad is really an evil Rogue from the future.
It's played straight with the Balam Alliance, though. You first have Oracion Seis, the weakest, then Grimoire Heart, which is even stronger than them, and finally Tartarus, a guild comprised entirely of demons from Zeref's Book of Demons and led by the very same demon who throughly kicked Igneel's ass in a fight.
Soundtrack Dissonance: Every episode ends with the credits music starting up a bit before the the ending credits actually start rolling. Considering that episodes tend to end on cliffhangers, this results in some...interesting scenes (read: really dark) running to cheerful music.
And with the third ending we have scenes like an incensed Natsu ready to fight Gajeel running to a maddeningly calm piano piece.
Most of the second opening is sickeningly sweet and is just general shots of guild members doing things that looks cool... until you get near the end where it shows the aftermath of Gajeel's attack, then Gajeel himself standing in front of shadow outlines of Element 4, and then Gajeel fighting Natsu, and the music is still sickeningly sweet.
Some people insist Laxus is Luxus, "Lux" being Latin for light, lightning; not "Lax," as in easygoing.
It's understandable though. Even if the official name translations from Mashima say that their names are Juvia, or Gajeel, or Erza, a check of the background shows that he himself seems to flip-flop on how their names are spelled. Erza's appears several different ways in handwritten form over the course of the story.
Freed/Fried and Bickslow/Bixlow. In the case of Fried/Freed, this might be due to the official translators deciding that Viewers Are Morons and thinking that people might mistake his name as being pronounced with the long "i" sound instead of the long "e". "Fried" is an actual name, as well as a common suffix for both surnames and first names.
Juvia is shown fighting alongside Gray (and others)before the conclusion of her battle with him during the Phantom Lord arc. This may not matter as much as other examples, since said fight makes it obvious she's vulnerable to a Heel-Face Turn.
Really the near-entirety of the third opening is spoilers, although aside from the above mentioned example you wouldn't figure them out yet.
You'd notice that the guy under the hood has the same tattoo and hair color as Siegrain.
Also, the second opening showed the destruction of the Guild Hall long before it happens.
The fourth opening is basically a montage of important scenes of the arc following the current one.
The fifth and sixth openings have Wendy standing in group shots that are otherwise only members of Fairy Tail. The fact that no one from Blue Pegasus or Lamia Scale is in any of these is a dead giveaway that she joins Fairy Tail later.
Subversion from the sixth opening: You see a young(er) Wendy walking around with a young Jellal, and she insists that he saved her life at one point, although he can't remember it due to his amnesia. It's actually Mystogan, who doesn't appear anywhere in the entire arc except for in Wendy's memory.
The eighth opening and ending spoils a lot of things about Edolas arc if you watch closely, such as the face of Queen Shagotte way before she actually appears onscreen, hints of the final boss of the arc, and the return of Mystogan. But the worst of all is the new ending heavily focused on Lisanna, who we barely see during the Edolas arc, and the friggin end of said arc, in which Lisanna run into her brother and sister at Magnolia's cemetery. Really? Spoiling the very last scene of Edolas arc? Really?! What the heck?!.
The tenth opening has a few spoilers of varying severity. Laxus is back, a brief shot of Romeo (who hasn't had an important role since Episode 2), A big dragon, Hades' "Demon" Eye, and a mysterious woman near the end. She's really Mavis, the 1st master of the Fairy Tail guild. And even more, her fading in and out next to and in front of Zeref implies the two of them have a connection. This doesn't happen until Chapter 340 in the manga, and it hadn't even gotten to that point when the aforementioned opening came out. Talk about foreshadowing.
A couple of the more minor ones include Gildarts fighting Bluenote (which wasn't really a surprise, because who else had the power to take Bluenote on?) and Natsu having an aura of lightning and fire in the opening shot (which doesn't really stand out unless you know to look for it).
The 13th opening shows you all of the guilds that got into the Grand Magic Games, including Raven Tail and Fairy Tail's B team. It also spoils some of the fights, including Elfman vs. Bacchus, which is supposed to be both an error and a surprise.
Even worse is that it has a scene for the battle between Jura and what appears to be Mystogan who has no business being there. 'Mystogan' is also using Jellal's magic.
At the end of the Grand Magic Games arc, Princess Hisui apologizes to Zirconis for dragging him to the present. As she does so, she mentions that she is named after jade, the color of Zirconis' body. He notes that "Jade Dragon" has a nice ring to it...
Staring Kid: Erza gets a crowd of them after accidentally breaking into a public bath.
Stock Footage: More powerful magics from recurring characters are given this treatment. Natsu suffers from this with every single move he intends to use more than once: Even his most basic attack, which is just a glorified punch.
Storm of Blades: Erza can do this by summoning lots of swords with her Requip magic. If she has enough time to set it up, she can even launch 200 swords at once, each set to hit a target across an entire town.
Strictly Formula: The anime endings before the Time Skip fell into this. All of them have: The appearance of a clock-like object somewhere in the ending, chibi versions of the characters, and a single character (usually the main focus of the current arc) running into either the background or foregroundnote The only exceptions are Mirajane and Gray, who both walk, and the former doesn't go away or towards the camera at all.
Stripperiffic: Around 98% of the characters use only a few garments of clothes at best in battle. After 100 chapters, if you find it weird (or impractical) to see people beating each other in only a scarf and pants, boxers, cocktail dresses, schoolgirl outfits or technically naked, you are watching it wrong.
Hoteye/Richard and Wally, who turn out to be long-lost brothers, both have impossibly polygonal faces. Wally's is presumed to be the result of his magic, since his entire body is polygonal, but who knows about Richard.
Ultear looks enough like her mother that Gray thinks he's seeing Ur when he first sees her.
Also Lucy,Happy, and Carla look like their mothers.
Strong Flesh, Weak Steel: Attacks that blow up buildings, leave huge craters in the ground or even rearrange the geography can have a mage survive standing in the centre of it with nothing more than bruises if the mage is tough enough or a complete No Sell for the truly powerful ones.
Stupid Evil: Cobra basically won his battle with Natsu. And yet Brain still pulled a You Have Failed Me on him, even though he didn't fail, apparently just because he didn't succeed well enough. You know, just in case we haven't hammered home that he's a bad person yet.
Stupid Neutral: The creators of Nirvana were a neutral country of peace who wanted to see the Balance Between Good and Evil preserved, so they invented a spell to turn dark to light and light to dark. Naturally, this falls into the hands of villains who think it'd be a great tool for tearing apart unity in the light guilds. Nirvana's creator explains that the "light to dark" feature was an unfortunate unforeseen side effect that made his own people turn on each other. Nirvana was only supposed to remove evil.
Zeref borders on having a multiple personality disorder. One side of him is very mellow, if not melancholic, and always concerned for the well being of strangers and creatures that just tried to kill him. The other side has no qualms with calling those who piss him off absolute rubbish, rubbing their failure in their face, and flat out murdering them. And of course it's the latter that has full control over his powers.
Summon Magic: One can use keys to summon Celestial Spirits. This is Lucy's trademark ability.
Inverted by Caprico, a.k.a the possessed Celestial Spirit Capricorn, who can summon human beings.
Super-Deformed: Pops up every now and then, especially in the endings.
Super Mode: "Dragon Force", the Dragon Slayer equivalent of a Super Saiyan transformation. Apparently much easier to induce if you're not an "old style" Dragon Slayer, but not as powerful as the real deal.
Midnight of the Oración Seis has the ability to reflect and distort things, including magic and light. So he can render himself untouchable, reflect attack magic back at the caster, apparently use Razor Wind, and even project illusions (by bending the light that reaches a person's eyes). He basically has a combination of the powers of Accelerator and Aizen. Too bad he can't use it directly on living things.
Although it's hard to set up, Fried's power is basically "make an area where anything I want to happen does". Lampshaded when someone says that's like a kid playing a game and making up rules as he goes along.
Wendy. Think about it. Dragon Slayers power up from eating their element, and Wendy's element is Air, which pretty much gives her a limitless supply of power in most situations. She hasn't learned most of her offensive magic yet, but the one spell she does know is shown to be very powerful. And if that isn't enough, she can heal. That kid is gonna be scary someday.
And now we have Chelia, the Sky God Slayer who has the same kind of lottery-winning powers as Wendy, only much more powerful.
Rustyrose can create anything he wants from his imagination. Elfman and Evergreen stood no chance against him.
Basically, the point being hammered home in the S-Class Trial arc is that to join Grimoire Heart at all, you are basically required to have won the lottery. Luckily Fairy Tail isn't a series where the only answer to Story-Breaker Power level magic is moreStory-Breaker Power level magic!
Many characters like Ultear have extremely powerful abilities under the catch that they can't affect living things (although Ultear can still affect plantlife). This is done so it won't be *too* blatantly broken, as completely freezing time around her along with Ultear's fighting ability, ice, and transform magic would make her pretty much unstoppable. She is still pretty close through the intelligent application of her power (nullifying spells and weapons to make the enemies defenseless). She was shown to reset time later, but at a cost.
Sympathy for the Devil: The conversation in Chapter 340 between Mavis Vermillion and Zeref implies that at one point, the two were close enough to know rather uncommon knowledge about one another; for example, Mavis knowing that Zeref is a Death Seeker, and Zeref referring to humans as if Mavis isn't one of them. She also apparently willingly let him stay on Sirius Island with her, indicating that despite Zeref's very well earned reputation, she seemed to genuinely feel bad for the guy... up until his rather unnerving threat about wiping out humanity, anyway.