Akdjgjkdgjkdfg it's SO Funny indeed, I'm thinking again about my AU where he meets with Tanjirou and Zenitsu earlier in the story, him witnessing Tanjirou using thunder breath in that context would be hilarious, he'd feel so much shame over it haha ^^..... and it would feed his self consciousness even more. I love the idea of Tanjirou being his "kisetsutai specialest little boy" oblivious self and making people around him self conscious.
but yeah in all seriousness i dont think tanjirou performed a perfect, full "first form" per se. but still... it makes you think about Why kaigaku could never use it himself. did he even try hard? not as hard as zenitsu but? still tried at all? or it's that he's someone who was so focused on reaching the top, envisioned himself only as The Best of The Best, he cut corners and never put his full attention into the basics...? to me it's the latter right! zenitsu and kaigaku's story being like the turtle and the hare, the one who went slowly but surely became victorious in the end... ^^

. but yeah. i want tanjirou to deeply embarass kaigaku while being completely oblivious about it. that'd make kai even more furious.
"See! See! Even I can do it! I'm sure you will eventually be able to do it too, Kaigaku! We can train together, if you'd like! I'll show you how I did it! First you position your legs like this...."
"Shut up and die!! Scum!!!!!!!!"
ah, the potential of kaigaku being exposed to tanjirou's irritating sunshine beams of positivity...

gentan! its cute! but when I try to think deeper into their relationships I always think genya would be self conscious around tanjirou and would never be able to go over it 😅 which is kind of sad. on the other hand tanjirou while being always well meaning could be very oblivious to how his attitude makes genyas insecurities worse, so I feel he could easily hurt genya without realizing it..... it`s sad......... but it's an interesting dynamic for sure. genya loves tanjirou but also has feelings of admiration, which sours into him comparing himself to tanjirou all the time and feeling miserable as a result. and tanjirou continues to be his little too good too pure main character self, genuinely loving genya but being oblivious to the hurt hes causing. yum

Frankly it could go both way! But let's say that I drew a lot of 'what if' kaigaku stuff that more or less fits into the story without disturbing canon, so these would end with his death ...
But this in particular is, definitely an AU, so we could be self indulgent and have Kaigaku redeem himself 😤 never turning into a demon at all, or managing to be cured, or living as a 'peaceful' demon like Yushirou for the rest of eternity 😤
I like the idea of him being a lifelong partner of Yushirou, i pondered this scenario a lot. Redeeming himself by living a long life and making sure the history of Kisetsutai is preserved - Kaigaku getting a sense of duty, a purpose that isn't entirely self centered if you will - in repentance for his selfishness and the trouble he caused (bc Grandpa still died in this I assume). So he was spared, and was gifted life when he should have died, but is cursed to see Zenitsu and everyone else grow old and die and carry the burden of their memory...😤
That or he stays human and Grandpa is fine and Kai grows old with Zenitsu and they kiss a lot 💕💕💕💕

....Or Zenitsu does kill Kaigaku as in canon, and lives his married life with Nezuko haunted by his failure to save Grandpa and Kaigaku. If I knew how to write it's a fanfic I'd love to write...
It can be a post canon scenario or written in the context of my Kai replaces Ino AU if you want more proximity between the zenkai, and a Zenitsu who must have felt very close to put Kaigaku on the right path but ultimately failed anyways...
So in both cases, Zenitsu grows older, and marries Nezuko. He wears the black uroko haori and keeps a magatama on a blue string as a memento. He loves his wife, but when he looks at her, he sees a woman who turned into a demon and was eventually cured. And he can help but sees through her, the face of a boy turned into a demon that He failed to save, and never had a chance to try and cure. Time passes, and the sight of his wife becomes unpleasant, because it mirrors his failures. His relationship with Tanjirou deteriorates, as Zenitsu's hard earned self confidence is slowly replaced with self consciousness. Perfect man Tanjirou, who saved his sister through pure sheer will, never wavered, never complained, is left with one eye and one arm. And he still doesnt complain, savouring every day of his hard earned peaceful life, working hard at housechores and cutting wood with his only functioning hand...
If Zenitsu used to admire him, little by little he starts to become really irritated. Because Zenitsu can't bring himself to work hard, can't bring himself to wake up in the morning, and cant even bring himself to look at Nezuko in the eye. He just lives his life progressively alienated from everyone else, haunted by the ghost of Kaigaku and all his failures....😚😚
.....that's a depressing story!!!! Maybe kaigaku can live and smooch zenitsu actually.