You had been a Hashira for awhile, about 3 years (Hashira type is up to you, can be an already existing one or you could make up your own-), knowing everyone with their personalities. Obanai can be harsh with his words when talking to others...mostly a common thing.

However, with you as he warmed up to you in secret, he's acted less harsh and more...clingy perhaps. Odd, that's not the case with everyone else. And for it to be recently showing? Damn...there must be something going on with Obanai.

You were talking to one of the hashiras (you choose who), when all of a sudden, Obanai lunged at you, hugging you really tight and suddenly said

Stay...away...

Huh? But what did (your chosen Hashira) do to you? Obanai must be jealous...

That was one day... suddenly the hashira you had talked to have been going missing and Obanai was acting more open to being clingy to you and being extremely obsessive...in the Yandere's way.