Abusive Parents: The main reason why he abandoned his mother, due to her went insane after he accidentally killed his father and his older sister later commits suicide.
Subverted later on when it's revealed that she wasn't Agito's real mother, and his real mother is WAY better than her.
Adorkable: Oh. HELL. YES!
All of The Other Raindeer: He gets bullied a lot by his classmates, even in Junior High. This ends up breaking him in Imaginary Shadows arc, having had enough of the bullying.
Almighty Janitor: Due to his Imaginary Numbers magic, Agito is by far the most powerful Level 4 Magician, and probably on par with a LEVEL 6 MAGICIAN.
Ambigously Gay: He can get awfully clingy around either Edge or his other male friends (but especially Keith), at one point ranting about how Keith has a better body figure to Viral.
He even kisses Roku in curiosity after Edge comments that Roku had probably kissed other guys before in the New Order arc. Hilarity Ensues.
Asleep In Class
Badass: After all, both of his real parents are a real Badass as well...
Badass Adorable: And he's also incredibly CUTE!
Crouching Moron, Hidden Badass: While he may be lack in terms of intelligence, but Agito really does prove he's a capable Magician and a Heritor.
Beat the Curse Out of Him: In New Order arc, when Edge ends up transformed into his "Beast" form, Agito gives him a good solid smack to his head that returns his body to normal... then he smacks him some more.
Beneath the Mask: Agito is way different than we all thought he was as proven in Imaginary Shadows arc.
Berserk Button: One of the things that can actually made Agito pissed off is someone who tries to hurt his friends (especially if its Edge) or insults his parents, and that's when Agito finally unleashed his true powers. Beat should've known it would come to him.
Beware The Silly Ones: He may be sweet and silly, but as Imaginary Shadows arc shows, he can be quite ruthless and almost downright insane when anyone already pushed him way too far, even if it's someone he is friends with (just ask Shintaro). And let's not get the fact he's also an Apocalypse Maiden.
Big Eater: One of the traits he inherited from Kisara.
Brilliant, But Lazy: While he doesn't normally show this, but Agito proves he's a lot more capable of figuring out things that is complicated even for the others to try (the Ouroboros Riddle), despite for his quite low intelligence. Even Daiki is amazed that Agito manages to solve the Ouroboros riddle in such a short time.
Break the Cutie: Suffered this a lot in the Imaginary Shadows arc, thanks to being Mind Rape a lot of times by Kano. This eventually what leads him to unleashed his Heritor powers.
Cerebrus Syndrome: You know the when story gets dark is when Agito is either in emotional turmoil or is not lighting up the mood anymore.
Chekhov's Gun: His Imaginary Numbers Magic is the clue in that Agito is actually both Teru's and Kisara's son, due to it being the Mutsuki's lineage magic.
Chewbacca Defense: Tends to use an Insane Troll Logic to most of the enemies, but particularly the Ouroboros, so much that he manages to make them stunned into silence.
Cloudcuckoolander: Takes it Up To Eleven at worst. He's a Cuckoolander so much that even it makes both Rachel and Xerxes quite bothered by him.
The Cuckoolander Was Right: Though there are at times where most of Agito's ramblings actually helpful. As seen in Akashic Records arc, where after he offhandedly mentioned Shintarou should sacrifice himself to save both Keith and Roku is what gave Shintarou the idea on how Roku will be able to activate his Akasha Seal.
Cool Sword: His Fireseal Sword.
Dangerous Forbidden Technique: His Imaginary Numbers magic, as long as Agito is capable of controlling it.
Dumb Muscle: While Agito isn't the brightest out of his friends but he is certainly one of the strongest one, right after Natsu even!
Casting A Shadow: His Imaginary Numbers.
Playing With Fire: His main Magic, Heat Fusion.
Fusion Dance: Can do this with Edge through Junction during the climax of Revenant Night arc.
Not So Weak
Sinister Scythe: Gains this as Ragna, his and Edge's Junction form.
Super Powered Evil Side: His "Imaginary Shadow" side is one for him, at least until Agito is capable enough to control it.
Unskilled, But Strong
Willfully Weak: He greatly holds back as the "Imaginary Shadow" while fighting Keith in the Imaginary Shadows arc and later in Agito arc, as he could easily destroy nearly half of the battlefield with his powers, but subconsciously fears killing Keith (and later Kisara) as a result.
Cooldown Hug: Gets this twice by Keith to surpressed his "Imaginary Shadow" form. He later gives one to the Berserk!Edge in the New Order arc.
Cry Cute: Tends to do this if it's not an Inelegerant Blubbering, especially as seen at the end of Imaginary Shadows arc and Agito arc.
Dark And Troubled Past: How do we start? First, just after he was born, Agito ends up getting taken away from his real mother and then is taken care by the family who kidnapped him in the first place. Second, things went downhill when Agito unleashed his Imaginary Numbers and accidentally killed his "adoptive" father and a few days later, finds out his older sister commits suicide leading to his "adoptive" mother going insane. Then, Agito abandoned his mother after having enough of her abuse until he met Edge and forms a contract with him. Even then on, Agito ends up getting bullied by all of his classmates just before he met both Roku and Keith. No wonder Agito manages to be a Stepford Smiler after all those things after...
The Determinator: When he's not in a real emotional turmoil, he always comes out of any situation with a smile on his face, no matter what. He's pushed to his limits when he is captured and almost getting killed by Kuroha, but still gave encouragement to Keith on his fight against him.
Then again, considering who both of his realparents are... Is not really surprising.
Distressed Dude: During Mary's Haze arc, as a result of Kuroha's Sadistic Choice against Keith.
Expy: Basically he's a male version of Mako personality-wise but his tendency to go through Break the Cutie moments are similar to Sakura from Fate series.
Fighting From The Inside: Keith thinks this is the reason the "Imaginary Shadow" won't kill him, since Agito is afraid of ended up killing Keith because of his powers.
Foreshadowing: Most of Agito's personality (happy-go-lucky, hot-blooded, idiot, a Big Eater, etc.) and tendency to rambling something nonsense foreshadows him to be the child of Kisara Sejren and Teru Mutsuki from Daybreak as revealed in Agito arc.
Freak Out: In Imaginary Shadow arc, after he ends up killing his classmates that bullied him (even though they kinda deserved it). Let's just say that Agito's mind is finally broken at that point.
From Nobody To Nightmare
Fun Personified: Long story short, this guy is a walking Crowning Moment of Funny. He always tries to lighten up the mood and every time he opens his mouth, you can be sure that he will say something crazy, funny, or (most often) both sooner than later. Even Edge said that Agito is the definite guy to make somebody laugh.
Although it could lead to a Deconstruction, considering how Agito's a Stepford Smiler and ends up getting his mind broken most of the times until his Character Development, where he ends up playing this Trope straight.
Generation Xerox: Most of his character traits are inherited from either Kisara and/or Teru, his real parents.
Go Mad From The Revelation: In Agito arc, after he was told by Kano that his "current" family is nothing but a fake from the begginning. Agito's mind at that time is probably weren't in his greatest condition so far.
Heroic BSOD: In both Imaginary Shadows arc and Agito arc. The former is when he ends up killing his classmates due to his Magic and the latter is when he accidentally hurts Keith, which both ends up with him consumed by his "Shadow" side.
Ho Yay: Has a lot of this with both Edge and Keith. Especially the latter. And that's not counting his Ambigously Gay aspects.
In the Blood: The more it's revealed about Agito's family, the more it's obvious that Agito inherited a lot of his personality from both Kisara and Teru. Heck, even Mukuro admits that Teru's rubbing off on his son a lot.
Let's Get Dangerous: Has his moments during the season 2 and 3 finale, but especially the latter when Agito manages to obtain Junction with Edge.
Living Shadow: Due to unable to control his Imaginary Numbers ability, he can morph into the "Imaginary Shadow" which happens whenever Agito is in an emotional state.
Mind Rape: Ends up getting this twice, by Kano no less!
Moe Couplet: With either Edge or Keith.
Mood Swinger: During the course of the Imaginary Shadows arc, his emotions and personality goes strongly unstable whenever his "Imaginary Shadow" is emerging.
Morality Pet: For Edge and Keith is this for him.
My God, What Have I Done?: After Kisara (his real mother) tells him the whole truth, Agito then finally snaps back to his own self and realizes that he's a danger to both of his friends and family.
Nice Guy: When he's not broken at least, but Agito is a genuinely a nice guy.
Nightmare Fuel Station Attendant
No Sense of Personal Space: Towards Edge and sometimes to either Roku and Keith.
Not Himself: In the Imaginary Shadows arc. Even Keith noticed that Agito is not smiling as he usually does and concludes that something is wrong with him.
Parental Abandonment: Inverted. He abandoned his mother after she went insane. Although it does end up in a good thing, considering she's not his mother in the first place.
Person of Mass Destruction: As a result of his Mutsuki lineage, Agito could end up like this just like his father but ten times worse if he's unable to control his Imaginary Numbers, which could end up destroying the whole city.