The MC is slowly becoming duller and I'm thinking the ex was one of the gfs who cheated on him, it would be cool if that were the case and if he would just flat out reject her, that would be an excellent outcome for this arc, since the only way for the author to apparently raise the stakes in the romance is to bring one of the 10 evil exes that want to destroy Megumi Pilgrim.
Either way, it's a light hearted romance but it's not a really good one, at least it's not on par with other mangas like Horimiya or rough sketch senpai.