fyozai idea 1
"I transmigrated into the body of the FL's older brother, and the obsessive yandere ML won't leave me alone!"
That's self explanatory enough but I'd like to think Fyodor(the obsessive yandere ML) would be wildly different from how he is in the novel, which is typical for these stories. He'd be some regressor bottling up a lot of hate for the OG cast including the FL's older brother. He's on his like 100th regression or something and he just completely gives up on everything and instead opts for this more spiteful route. IN WHICH, he gives up on taking care of his health for the majority of his youth. In the "original story" that I'm half making up rn, he wasn't as sickly, and took care of his health regularly with a lot of due diligence. So, in this story he's more frail.
When the recently transmigrated Dazai meets the regressed Fyodor he specifically notes that he looks nothing like what was described in the novel and looks scrawny(they meet as kids), and wayy more hostile towards transmigrated Dazai and his sister. BUT Fyodor quickly realizes that Dazai in particular is acting out of character. There's this mutual realization that something is up, their reactions vary a lot though. For example, Fyodor becomes really curious and a bit desperate, while Dazai is instead more intrigued and confused.
OKAY, so now it's timeskip!! Academy arc ect, all that good stuff. If their childhood was this mutual curiosity then their teen years are a whole lot of antics and just trying to get the other to crack. Fyodor is still inherently yandere and possessive ofc, even more since he(Dazai) is the only one acting differently. Similar to a beacon, everyone else in the cast is also beginning to act differently because of Dazai and Fyodor is even more protective of him as he's hoping Dazai will be the key to his escape from the regression loop thingy. Dazai on the other hand is beginning to get really irritated with Fyodor since he can't "read" him like the rest of the cast.
Romantically, I think Fyodor would fall first purely because of his obsession, while Dazai is one of those main characters who still views the world they transmigrate/reincarnate into as a "book" even though they lived something akin to a second life there(being there since childhood). Fyodor was the only person there who actually brought him down to earth, and actually made him "think"(he is typically detached from the moment).
In the end, they learn the truth about the world and each other, and then die together<3(after they fix fyodors regression thingy and kinda destroy the world)