I have her. I love her VA, and if I was danchou I would instantly put a crossbow bolt (seems all the guns in FKG world are magic guns, rather then science derived, black powder slug throwers) straight through her head.

I had her for a loooooong time now, the normal version on Nutaku and the bride version on DMM, so while not a perfect comparison, at the same time having the bride version I can somehow compare, and I completely agree on the rating of C-

She needs some buffing to be done as she struggles in every aspect. Not much of a offense to work around her stronger dmg buff, and not much of a defense to make her "if hit" skill of any relevance.
She has guts, which is at least something to make her work, but honestly, with her low Defense it doesnt come to play as often.
Her stats are attrocious, exemption being the speed which is very good.
Skill is the basic AoE, not even sure why people are rating these separately, to be honest as skills are all 4 paper cut types for everyone.

Overalll she is mediocre at best, borderline subpar at worst.
Giving her Evade, just like the bride form might help a bit as Evade and Guts have proven to be a solid combination.
She is also one of the few girls who could benefit from Skill Damage buff with her bonus skill act if hit. But it would have to be more than the usual 15 to really make a difference. 20-25% would be something to actually raise her ranking.

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With that being said let me get to the juicy parts which are non rating related. Game already covered pretty much all of it. I had made a long post in the old FKG forum part where I took a deep mental analysis of all the yanderes. What he wrote pretty much corresponds to my analysis.
Lavender is one you could tame, coz she loves you so much you could play around that to make sure all is well, Suzuran is like a child, so while you could teach her to a degree she would still be unpredictable. Dangerous.
Victoria...... yeah this chick is a fucking psychopath/sociopath by definition. Danger level maximum. Kill on Sight less your survival is seriously compromised.
When I watched her bride form H scene I was hoping that maybe, just maybe she might be a bit more.... caring ? Less mental ?
I cant read the text but I get the feeling that was not the case and she is pretty much the same. There is no emotion behind that plastered smile of hers. She is an empty void inside.
Do not engage ! Eliminite on sight. If I was Danchou in real, I would constantly keep Mei and Sakura or some of the other powerful duelist and seasoned veteran girls around me as bodyguards.

*Now as for the art the basic form is .... meh. That lily pad skirt is just bad, and that bikini bra doesnt help either.
Her secondary artwork is much much nicer, and the third ones is also very good (its a discounted bride version one though).
She is definitely a looker.
Unlike some other girls, her axe is as plain as it gets. Its literally a lumberjack axe, as simple as it can be. That being said, I think she is the only axe wielder in the game, so thats something. Axes are dope (insert obligatory Gimly meme )

Her voice......haaaaah... yes... her sweet amazing voice.
Oonibasu Voice Actress did a stellar job, perfectly fitting both the artwork and the mental state of her. Her usual voice is a fake melodic, cold and uncaring. When she gets heated it gets a higher hysterical tone to it to show her broken mind and lack of empathy.
Fantastic job on the VA actress, I love this voice over and wish she did more than just Oonibasu and I believe one 5*.
I believe it was Taon who took the time to dig around and managed to find out info about the VA, her other FKG characters, and also other non FKG related VA work. I think its somewhere in our Chit-Chat thread.... or maybe in the Nutaku one. I am not sure, but its there somewhere.

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So yeah Final Verdict C-
Oonibasu was fairly popular for a time, so I its a shame that she doesnt reflect how amazing of a character she is (amazing in as - how her character is written, superbly voice and has an overall fitting theme quite unique to the usually cheerful character of the FKG world. ).
Certainly some "buffing" would be in order to bring her up to speed (I cant make myself type "loving" as she is unable of such a thing and does not deserve it in return either.) with the new challenges and stronger enemies.