Gasper Vladi (Highschool DxD)

Ah, the crossdressing Vampire of the Gremory Peerage, I can see that in a few situations.

Maybe he wants Issei to love him too so he either makes Issei feel that way or he uses his new powers to turn himself into a girl.

Maybe he's jealous/angry that the girls of the peerage are paying attention to Issei and decides to change that because he was part of the peerage first and Koneko-chan should be his not Issei's just like Rias' hugs and love should be too.

Maybe an enemy or someone wanting **** places a magic on him so he'll take all the women from Issei and/or Make Issei miserable as he loses his harem to a younger boy.

Maybe his vampire instincts kick in and he wants to claim brides to be beside him forever and who better than his friends.

Maybe it's a Gasper from another timeline and his Balance Breaker let him get into a new timeline and he doesn't like the Gasper of this timeline and decides to take over and have fun. Doesn't even have to kill the current Gasper, just make him into a girl (or leave him a boy if they're into that) and claim her as his first conquest before aiming at the other females.

Maybe Issei's perversions have rubbed off on Gasper and decides he wants to be a harem king and replacing Issei is the best way to do it.

There's no doubt dozens of other options too.