Sena Izumi is a kid with far too many secrets and shadows for someone so young--having skeletons in your closet the size of Volkswagens seems to be a prerequisite for most characters appearing more than three times in this series, and Sena fits the bill on all accounts. Working for Kritiker, a covert organization existing outside of Japanese law, from an unstated age (likely no younger than 14 or 15), Sena found himself assigned to a new team he had no place being on. A sixteen year old, little-trained and untried, in Weiss--Kritiker's highest level team, and its only lethal team, at the start of a new and extended mission that saw two of the group's usual members off in Europe, thus the need to replace them. Sena wasn't alone, in fact the other new member of Weiss, an older teen named Kyo Aguri, was also placed on the team under the command of Aya Fujimiya.
Both Sena and Kyo had close ties to the mission at hand. Sena was just quieter about his. Placed in Koua Academy as a underclassman student, Sena let on little about his past, even if he likely wasn't fooling anyone. The mission: To determine the exact cause for the rash of suspicious suicides among the student body, uncover the link between the school and a rash of terrorist assassinations occurring worldwide, while looking for any information they could find on the presumed ringleader of Esset's latest pet project--Epitaph.
Sena entered the school pretending to be just another straight-C student, Aya was a bespectacled history teacher, and things started with a bang (or a thud, rather) when Sena tried to prevent a girl committing suicide and failed. He got his first taste of the appalling apathy of the school body when they immediately questioned why he bothered, but he also managed to make a friend--Nozomi Miyazawa.
But weirdness soon resumed, as a troop of white-uniformed students burst into the middle of class, pulling out a student for having 'unstable brainwaves.' The Lifestyle Guidance Council, as Nozomi was quick to inform Sena, controlled all the activity of the school and even had more power than the teachers. The student had, until just before Sena's arrival, been a part of the elite S Class, a group of students that basically ran the rest of the school, keeping order and preparing the next generation of 'leaders' through brainwashing computer programs.
And Nozomi wanted to be part of all that, as she told Sena after running into him at the library the next day (staring forlornly at a picture of a blue-haired woman in a book marked Kisaragi Academy...Foreshadowing, kids!) Despite knowing full well what the academy was all about, Sena had to refrain from warning her off and instead go quietly back to class--where the boy from the day before promptly freaked out and ran screaming from the room. One dramatic eyecatch later, Sena (and precisely no one else) took off after him, catching up with the boy when he was just about to jump from the roof. Sena tries to talk him down, but is foiled by the arrival of Todou--the leader of the Lifestyle Guidance Council, and therefore the defacto leader of the entire school--who proceeds to needle the boy into jumping. Sena somehow saves him anyway (one-handed, despite the fact that the suicidal boy had a good bit of height and heft on him, and Sena was halfway across the roof when he jumped, somehow not dislocating his shoulder in the process) and proceeds to threaten to punch Todou's lights out, asking just where the hell Todou got off on being such an ass toward his fellow students. (Todou proceeds to be a complete Stepford Smiler, speaking in super polite Japanese even with a pissed off, shouting Sena about to rearrange his face, but that's neither here nor there.)
When he gets back to class, he finds out that Nozomi has been accepted into the S Class. After a brief interlude with his not-girlfriend to set up the drama, Sena is sitting in English class with his mind thoroughly not on verb phrases, instead thinking about the mission and the information Weiss has collected thus far--realizing that what they need has to be in the S Class. Spotting Nozomi outside the window with a look like someone just hit her puppy at seventy miles per hour in a flaming Mac truck doesn't do much to dispel any bad feelings about said group.
She doesn't look much better when Sena 'conveniently' runs across her during S Class's free time. In reality, he was sent by Aya to get information about the S Class from her, who keeps watch on the two of them from the treeline along with Kyo. And some five or six unnamed thugs in black school uniforms and creepy masks. Sena (also fully aware of their audience) proceeds to play her like a fiddle, promising to protect her, out of three-fourths sincerity and one-fourth necessity. Just as she bursts into tears, sobbing about not wanting to go back to the S Class, the masks burst up from the bushes and things proceed to get dangerous. Sena takes out four of the punks all by his scrawny little self before Aya jumps in to chase them off, with Kyo hot on their tails. Nozomi, for her part, passes out like the good little damsel in distress she is.
Nozomi gets transferred back to the normal class with Sena, and there's barely time for her to get back in her seat and compose an email for Sena before the subliminal messages programmed into the computer screensavers come on. That email would turn out to be her last words--she bolts from the chair and runs for the window. Sena tries desperately to talk her down, but he's unsuccessful--and he barely misses catching her as she jumps, unable to do anything but watch helplessly as she crashes to the ground. He proceeds to go a little bit more than off the deep end, claiming to anyone that will listen that Nozomi was murdered (which technically...) even though all the evidence, and a classroom full of eyewitnesses, pointed straight at suicide. Unable to convince anyone that could do anything about it that Nozomi's death wasn't a suicide, Sena went back to the site of her death and tearfully apologized for being unable to protect her--and dropped a small bombshell when he asked no one in particular the question: 'Did you mean for the school to be this way?' ...and directed the question at his absent mother.
...in the meantime, Todou somehow deduces that Sena is in Weiss because he could beat up a few muscleheads, and puts him under surveillance. And also begins to hatch a Grand Master Plan when the assistant director of the school calls him on some of his bullshit. This is all relevant, I swear.
In Weiss's basement, Sena proceeds to fly the hell off his handle at Aya, Kyo, and Rex--still insisting that Nozomi was murdered, and professing his desire to find whoever did it and shank them with extreme prejudice. Kyo floors him for his trouble and leaves him there to get expositioned at, revealing Kyo's angst and drawing a parallel between Sena and Kyo's dead little brother, one of the previous suicides at Koua. This lesson does not stick with Sena at all. He drives off toward Koua when it's revealed that Weiss's security has been breached, to investigate for himself the S Class dorm. He runs into a macabre funeral procession instead, and confronts Todou there. The younger student proceeds to play Sena like a well-tuned violin and send him after Assistant Director Inagaki. Sena performs admirably, offing Inagaki's bodyguards and fatally injuring the man himself--all outside of Weiss's mission orders--and gets a great shock when Inagaki looks him straight in the eye and says his name. His real name. Sena recognizes far too late that the corpse leaking blood onto the carpet is his uncle.
Of course, a kill outside mission parameters is nothing but murder. Inagaki's death gets Sena in some serious shit, and prefaces everything at the Academy blowing up rather spectacularly. Todou, springboarding off his manipulation of Sena, takes over the entire Academy while Sena and Kyo have a heart-to-heart. Kyo leaves Sena to have angsty flashbacks while he goes off to try and salvage the Koua mission with Aya...and, well, Sena proves rather handily that he has hefty angst to back up his issues when he recalls cowering in a closet, all of ten years old, in absolute terror as his sister and father are killed by the same woman he was looking at a picture of barely a few days ago--his missing mother--who then turns her gun on him. Somehow this inspires him to go running off to Koua and straight into yet another of Todou's plots, ending up with the entire school populace chasing after him in a bid to get into S Class--a plan to lure out the rest of Weiss. It works to lure out Kyo, but Sena, injured, can't do anything to help him and is forced to run. He has to be saved a second time by Aya, and together the two of them go to help Kyo. They get sidelined by a student from the newly revealed Z Class, made up of genetically manipulated, superpowered teenagers, and promptly get their asses kicked around the schoolyard until Rex (the secretary) shows up to save the day.
Unfortunately, they're too late to save Kyo, and Sena is struck incredibly hard by the death of the older boy, just days after losing Nozomi. Aya tries to comfort him and warn him off in turns, but Sena stays, and everything at Koua seems to return to business as usual, even with the return of teammates Ken and Youji from abroad. Due to the compromise of his position, Sena is left on inactive status--his punishment for killing Inagaki postponed indefinitely.
Even though Sena was on inactive status, he continued investigating the disappearance of his mother, questioning people who worked at Kisaragi Academy. His actions, even on his own, were precisely in line with Persia's plan to draw out the person or people controlling the Academy and all the experiments inside it. However, everything in the Academy seemed to be dying down--and Weiss is eventually ordered to leave the Academy completely. They refuse, and continue the investigation...Sena runs off half-cocked again, after Shimojima this time, but ends up running his way straight into a problem a lot bigger than he can handle--the Wonder Twins that kicked his and Aya's ass a few weeks ago. He gets turned into a human wrecking ball in short order, and not in any kind of a good way. Injured, he still continues to fight for the honor of Weiss and the memory of Kyo until he gets punched into a stone surface one too many times and collapses until Aya swoops in to save the day. Sena promptly passes out and has yet more nightmare-flashbacks about his past, the murder of most of his family that prompted him to join Weiss in an attempt to find the truth behind what had happened that night.
After he's healed from the beating he took, Sena continues assisting with Weiss's investigation--getting more clues as to what exactly happened to his mother when he sees a very familiar silouette in Ken's surveillance monitor.
Enough information gathered for Kritiker to act, Persia--Omi Tsukiyono--gives Weiss the mission to destroy the laboratories creating artificial humans, and eliminate Tsuji and Shimojima, the presumed heads of the entire operation. After a power montage in true 80's fashion, Weiss heads off to kick ass and take names (Sena actually does rather well for himself, since Aya used up his contractual Big Damn Hero moments.) Long story short, Shimojima offs himself rather than choose between death by telekinetic and death by sharp stabby things, Youji has a breakdown, and Weiss splits up--Sena runs off into Esset's doom tower (followed by Ken and Omi,) but not to find Tsuji...he says instead that there's something he needs to find for himself.
First on the list of fun diversions, Sena, Ken, and Omi run into a pyrokinetic and spend a bit of time going 'shit how do we kill this guy' before Schuldig (Gluhen is the land of nonsensical cameos) shows up and shoos them on to face the next challenge. In the next room they run afoul of another psychic--and Sena finally finds what (or who) he was looking for--his mother, Fumie Kisaragi...Epitaph. However, before he can follow her, he's caught in the psychic's illusion and fights with Ken and Omi for a bit until they get saved by another random cameo, immediately going after Epitaph. But because Tsuji isn't done screwing with them yet, they run into more of the Z Class superpowered teenagers. Aya manages to catch up shortly after, and his intervention gives Sena an opportunity to run off after his mother in truest half-cocked fashion.
He finds her, and tries to ask her why she killed his father, his sister, shot him, disappeared...tries desperately to get her to recognize him. The only response he gets is a raised gun and a bullet to the chest. He dies in Aya's arms, apologizing for being unable to kill his mother.
He then woke up in Somarium, the first time around--got himself in trouble a few times, snarked back and forth with Youji, and started to get closer to Aya-chan--based in the fact that they were both somewhat outside the bonds of Weiss and yet still within the circle. It's complicated.