Won't go into too much details. But his old gf does eventually join the harem. The situation is kinda weird, but let's just say the she was deceived and it wasn't completely her fault for what happened. Although if she had shown more trust to the mc, it wouldn't have happened. The mc is also a huge idiot for not trying harder to convince her, he even blames himself for that too. And no, she isn't in a relationship with the other guy, she basically treats him like a shoulder to cry on. The other guy does try to kiss her once to advance the relationship, but she rejects him cause she can't move on from the mc. Author had to make sure she stayed pure otherwise we can't be having her join the harem