Oh boy, I'm tried to search through the index, but I can't find it. For one thing, I haven't read any manga in about a year and a half, so I have no idea if this manga has even updated in a really long time.

I'm pretty sure it's about a girl and guy who pretend to date to get the attention of the people they really like? I think the girl falls for the guy first, and then he's revealed to be really cruel and mean, and I don't mean like a kichiku or an annoying person who's still kind of played off as fun. He was like a proper asshole who was really awful. That's the only plot point (bit of characterization?) I'm certain on. He also has black hair. It had maybe 10 to 15 chapters out when I read it last. Oh, and the art was a bit more shounen than shoujo, if I remember correctly. It had clean lines and inking without a lot of screen tone. It might have run in GanGan or GanGan Online...? Maybe?

Kuzu no Honkai? - If it is, then a warning. The ending sucked.

No, but I did read part of that one. No need to finish it, I guess! I really hope I'm not mixing up memories of multiple series that had insincere relationships.