First, welcome! Second, while Burgundy already touched on it I do feel like the 100% winning streak could be realistic, if played out properly. Raibaru (just...f**k that name) looks cute, sweet, and innocent, not at all what you'd expect from the "Indestructible Rival" (also wtf, you generally don't call people indestructible? at least not in this context...idk just sounds forced to me but anyway). HOWEVER--what if she wasn't as cutesy, as down-to-earth and girly? As a former martial artist, I've known my share of talented kids who get pressured into continuing by their parents, senseis, or their peers. Maybe Raibaru was a gifted child, a kick-ass martial artist who dominated in competitions and in school clubs, whose whole childhood and life led up to being the best martial artist she would be, no second-best, nothing else. She isn't allowed to be an average little girl, or to try other things--drama, cooking, video games--all she knows is training, competing, martial arts. Maybe, while she loved the sport at first, she grew bored of it as she got older, or maybe just wanted to pursue it as a hobby, rather than as a competitive/professional lifestyle. But she can't quit--her parents will be disappointed, she doesn't want to let her friends or teachers down, she feels like she's got something to prove, so she sticks with it, wishing that she could lose but being too proud to throw a match.
Then Budo comes in. He's learned from the best, and he wants to prove it. Raibaru is the best, and they both know it, but he knows her moves, knows how to beat her as no one else can. And then it's over. Raibaru's been beaten--she's free. She accepts her loss, turns the club over to Budo, and for once is able to be true to herself. She isn't the badass anymore. She can be an average girl--no pressure, no forcing herself to fit a mold. She becomes the cutesy, adorable character the player comes to know, still with a plethora of martial arts knowledge and skills, but now also with an admiration for the friend and pupil (and maybe, if the right cards are played, something more?) who bested her and who made all of this possible.
That is character development with a smidge of thought put in. Cliche? Admittedly, but not dreadfully so. It fits what Yandev intended for her backstory, and if you wanted you could toss in why she likes Osana so much--Osana isn't afraid to be herself, she's loud and girly and takes shit from no one, and Raibaru likes that sincerity, so they become friends. Or something, idk. Point is, it isn't that hard! If you think for a minute, you can put the puzzle pieces together in a way that makes sense and is still enjoyable. Hell, I'd even argue that it makes sense for a martial arts master to take you down if you attack her/her friend.
BUT, as always, YanDev ruined it by making her ridiculously OP, being a stalker as opposed to a considerate friend, and being shoehorned in because he was too lazy to change/consider making changes to an asset that ISN'T HIS that he is still claiming as his own.
Honestly, if we didn't already know how homophobic he is, I'd say Raibaru has a crush on Osana, is jealous of her affection for Senpai, and could either A) be a matchmaking option/suitor for Osana or B) Be an ally for Ayano, albeit a pacifist one, in which Ayano works with Raibaru so that they both end up with the girl/boy they desire. Buuuuuut YanDev would never consider that, since he seems to hate anything gay unless it's in his personal hentai collection and involves animated girlxgirl action only.
This made my stomach turn. The way he said it, just--ugh . TBH we've all had fictional crushes at some point in our lives, but the fact that you're imagining taking the virginity of a girl who doesn't exist, and that you'd be unable to view her as anything more than "dirty and impure" because you weren't her first everything is disgusting. Like how effed up do you have to be?