> non typical
If you're basing it upon other stories and you know it, in no way is your plot 'non-typical' (note the hyphen; "nontypical" is also an accepted spelling.) Just by glancing over it, I can tell you that the essence of your story was done years ago in a related medium by the erotic game (later adapted into an anime) "Kimi ga Nozomu Eien"- only it settled for milking the tragic twist once instead of twice.
> some comical reliefs
The term is "comic relief." The word "comical," as an adjective meaning "relating to comedy," is obsolete and has been replaced by the related word "comic." Comical now describes specific characters or occurances that inspire amusement. If your meaning was that the plot developments provide such relief they inspire readers to laugh out loud, your altered phrase may be apt, but I have doubts.
> Keitaro who just got admitted in highschool after being adopted by her only family her aunt with her 1st cousin.
I thought you said that Keitaro was male. Anyway, high school is a rather late age for adoption. What caused this turn of events? Did she live in an orphanage previously, or with other foster parents?
> Growing up alone he decided to rely on his own strength to protect himself so he studied swordfighting unorthodoxly, another of his special trait would be his ability to copy others almost instantaneously and perfectly may it be their sport or form in kendo.
Nobody "grows up alone" in a first-world country. Put some thought into the aforementioned question. What is unorthodox (the word is not typically used as an adverb) about her study of swordfighting? What grants her this unparalleled talent for imitation, and why has she not applied to other situations, such as social ones?
> sempai
This is not an English word and perfectly serviceable translations, such as "upperclassman", exist. Why did you use it?
> Well, it implies that both the female protagonists fell in love with him at first sight
If Keitaro was "shy, reserved and unsociable" why would anyone fall in love with her at first sight? Do they have a fetish for wallflowers?
> Katsura has a degenerative condition that would take her life before their monthsary.
"Monthsary," although a locution that has inarguably seen use, is not in any reputable dictionary. I would advise against using it.
Moreover, you seem not to understand what a "degenerative condition" entails. It implies a slow decline in health over months or years, common examples being cancer and Alzheimer's disease. If Katsura's condition is indeed "degenerative," she would almost certainly be visibly in ill health, and unable to attend school or form a relationship with Keitaro a mere month before expiring; she would also be exhibiting borderline sociopathic behavior not to inform him of her condition and limited life expectancy.
> literally self destructing.
What is literal about it? Are you implying that she spontaneously combust? This seems at odds with the ongoing description of Keitaro as alive and well.
> traditionary
While this is a word, it has not been in common use since the 19th century. I doubt it is the one you meant to use.
> But then again, a certain event would take Nayuki's life (a car accident maybe)
As I mentioned previously, this is exactly the same tragic event as occurs in "Kimi ga Nozomu Eien." While you may not have been aware of that, your plot is nonetheless unoriginal. Moreover, suddenly killing off two love interests through two unlikely turns of fate is putting your readers' suspension of disbelief through a harsh trial. Is this the only way you can think of to create dramatic tension?
> As a defense mechanism, he becomes a shell of himself and just copies personalities of others to mask his extreme sadness.
Why did she not do this earlier when she was unsociable?
> and btw i'm not japanese
That is obvious. Out of curiosity, is English your native language?
In summary, take a writing class.