Overview
Nanami is an overwhelmingly powerful combatant who's held back by her body's inability to cope with that power. As a result she's weak and sickly, her body beset by frequent and painful illness throughout her life. And yet her body refuses to simply die, leaving her to suffer through it. Her amazing visual acuity allows her to copy techniques and abilities she's observed, something she's already used to mimic her family's martial arts and several ninja techniques. Her goal is, in short, to die so that she does not have to suffer with her weak constitution any longer. Nanami's personality is calm and kind on the surface, but underneath that she's actually quite bloodthirsty and fatalistic. Furthermore as a result of her social isolation, she's not very good when it comes to social situations.
Perks
Kyotoryu: A martial art countering armed opponents which disarms or breaks foes' weapons.
The martial art that the Yasuri family practice, Nanami learned it without being trained. Instead she simply observed her father and her brother using it. It's a martial art that is sometimes described as swordsmanship without a sword, designed to fight against armed opponents using techniques based around disarming or simply breaking weapons.
Migeiko< Copy-T >: Nanami's eyes let her observe and copy the techniques of others.
Nanami was born with visual acquity so good that she can observe and copy the techniques of others. She can learn it by viewing it once, and deepen her mastery by seeing it a second time. Additionally, her eyes allow her to understand the stances of combatant, allowing her a degree of knowledge of what's coming her way.
Ninpo: Tsume Awase: A ninja technique that grows the fingernails into long, sharp claws.
A ninja art copied from a ninja that attempted to kidnap her. Using it grows her fingernails out into sharp claws, granting cutting power without needing a blade.
Ninpo: Ashigaru: A skill rendering her and her things weightless; enough so to walk on water.
Another copied ninja art. This technique makes her body and things she carries effectively weightless. Among other things this allows her to walk on water, and keep opponents in the air with little effort with repeated strikes.
Ninpo: Makibishi Shidan: A ninja technique that fires small, barbed, poisoned projectiles.
Her third copied ninja art. Using it allows her to fire poisoned, bee themed caltrops. They're barbed, and tough to pull out! The poison, if allowed to spread, causes numbness. Not fatal by itself unless delivered in extreme quantities, but a hit or two will slow the target down and generally be quite debilitating.
Akuto Bita: A blade that can forcibly rejuvenate and heal its wielder.
The Deviant Blade that Nanami stole after killing its guardians, this kunai-like sword is used not on its foes, but on its own wielder. When it pierces its victim, Akuto Bita forcibly rejuvenates its target, which compensates for Nanami's weak constitution and allows her to fight through grievous injury... but it still /hurts/.
Uncanny Strength: Her physical strength far exceeds what you'd expect from looking at her.
Thanks to her eyes, Nanami was able top copy the strength of the Itezora clan. As a result, taking a hit from her can do far more damage than you'd think.
Complications
Deviant Blade's Poison: Deviant Blades pollute the mind, making her very possessive of the one she has.
All of the Deviant Blades inspire a territorial kind of possessiveness in their wielders. Akuto Bita is no exception. Nanami will not give her weapon up willingly, and any whiff of someone trying to could drive her to behave violently.
Frail Constitution: Nanami is prone to bouts of illness, something she'd really prefer to avoid.
From birth Nanami has always been sickly. It's quite common for her to suffer from illness and disease, and if something's going around it's going to be a bad time for her if she catches it. She'd prefer to avoid the possibility altogether. If there's a situation where she might get poisoned or catch some illness, she'll likely try and avoid it. By forcing somebody else to do it if she needs to.
Remorseless: Nanami has a distinct lack of empathy for others.
When there's something Nanami wants done, things like morality don't generally enter into the equation. As a result, it's entirely possible that her actions might have garnered her a few grudges.
Desirable Possession: The Deviant Blade that she holds affects others as well.
Akuto Bita's poison doesn't just affect her, but tends to get into the heads of those around it too. While the more noble and just sorts might resist it easily enough, the greedy or power hungry might seek to possess the blade themselves. Having it in her possession tends to attract thieves or inspire the weak willed into trying to take it from her, no matter how much of an obviously bad idea it might be.
Deathwish: She plans to confront powerful opponents until one of them kills her.
In short, she's pretty much looking to throw herself at powerful opponents until she dies. In practice this is likely to lead to her escalating a conflict or even outright attacking somebody that was able to be dealt with relatively peacefully. When the villain make a terrifying display of power, gloat about it and prepare to spare the party's pitiful lives, Nanami is the one who'll start attacking. And then everyone else has to deal with the fallout.