What is that girl talking about? I don't remember our resident psychopath MC being interested in anyone else than the girlfriend who never appears anyway.
Let alone marrying another.
@Nehona I think it's the girl that he promised he'd give her a good life...maybe she took that as a marriage proposal. Also, he's shown interest in other women...just not to the same level of the one he's engaged to now. Like the cousin or whatever she was...the one that broke his heart in his past life. There's also that other girl that died in his past life(which was never revealed how she died or how he'd stop it from happening this time).
And never appearing? His fiance was just shown a few chapters ago with him...are you reading properly?