I often read your MHA fanfics involving DFO and the relationship between AFO and First. I really like them. Yandere AFO is so spot on, but I also liked yandere Yoichi. But what about if other characters were yandere? So I want to propose you an idea about yandere Second and yandere Third with Yoichi as darling if you want to write it. You have already made a fic about Yoichi being possessive about Kudo and Bruce [Carved in Stone (Eroded by Water)], so why not the opposite?
Just imagine the heartbreak from Yoichi when he realizes he traded one yandere for two!
My idea invvolves Second and Third becoming yandere for First after rescuing from the bank vault.
At first they don't display yandere tendencies, but do after falling in love with First and fearing constantly his capture from AFO. They try to be much more subtle about it than AFO. The two try to prevent Yoichi from going out of their base with several excuses, until the latter finally understands what's going on. At that point, they try to guilt trip him, but it doesn't work, so they think they have no other choice but lock him up in their base. Their yandere tendencies are now open, but different from AFO, because they hope a slower pace would help him see reason/fall in love with them (translation: develop Stockholm syndrome).
For starters, they give him more space:
1) when Yoichi cried for the first time (due to the pain of betrayal), trey tried to hug him, but he rejected them, so since then they let him cry until he stops and just silently offer handkerchiefs; in fact, they avoid initiating contact unless Yoichi does first (including the sexual kind)
2) no infantilisation: they let him eat, dress, shower...etc by himself; they only stay near to be vigilant of any escape attempt
3) they let him sleep on his own bed, though they will sleep in the same room to be vigilant of any escape attempt; they only sleep with him when he starts "warming up" to them
4) speaking of escape attempt, if he tries to, they might chain him; however, they wouldn't really go further: they would rather deal with multiple escape attemps, no matter how headache inducing, than to put him in inhumane conditions, speaking of which...
5) NO BANK VAULTS OR DARK ENCLOSED SPACES: seeing Yoichi broken in the bank vault really hurt them, so they would rather keep him in large illuminated enclosed spaces, or if not possible, they would avoid leaving him alone so he doesn't suffer from isolation, even using other people of their vigilante organization. If Hikage is with them, he's an accomplice.
Also, if/when he tries to hunger strike, UNDER NO CIRCUMSTANCES THEY FORCE FEED HIM. They make deals instead, like letting him decide which AFO's bases to destroy or letting him shop animal food online so they could give it to animal shelter. Basically they bribe him with the possibility of helping people or animals, even if indirectly.
They also let him excercise, so he would have more chances to escape AFO if he finds them. Kudo and Bruce exercise with twice effort to protect him and, of course, be able to catch him if he escapes. They won't let him near object he could use as weapons, though.
Kudo and Bruce are still vigilantes, so they wouldn't murder the first person they find to make Yoichi comply, only if they are a threat to his wellbeing. That being said, it doesn't mean there aren't buttons that mustn't be pushed. Yoichi learned the hard way when in anger he compared Kudo and Bruce to AFO. In tranquil fury, they asked Yoichi to take it back. He didn't. They decided to MAKE HIM take it back and apologize (translation: they tickled him until he gave in).
Besides all of these implementations, they also try to bond with him sharing their tragic past/experiences (up to your imagination).
With all of this, Yoichi eventually gets attached to them. He thinks he has no one else and this might be the best he can get without being too smothered. The three begin to sleep in the same bed and might even have sex.
I hope you like this. If you think the actions are not really in character for them, you can change them. My idea is to make them possessive, obsessive and overprotective, but no sadistic, in order to differentiate them from AFO. They still haver more empathy than he could ever have. Nevertheless, if you think I made them too soft, I understand if you want to change that.
If you don't like the prompt, I'm open to being redirected to someone else.
This is a fun idea! I should give Second and Third a turn to be yandere. I'll write something like that one of these days, after I finish up my current projects.