The Main Heroine Likes the Extra

He is the fiancé of the sub-heroine and an extra who appears briefly as a villain early on.

Despite his self-discipline and amicable relationship with his fiancé.
She went to the protagonist’s side.

Author note:
There is a description reminiscent of NTR in the beginning. It has nothing to do with the heroine, but please be aware that some people may find it offensive.