That was a pretty rushed end and quite the large information dump, it feels like it could've used another volume or two. It's rather disappointing the childhood friend won when she ruined his life and other girls were so much more well-developed. I'm also left wondering about the childhood...
I decided to look into the web novel and it looks like the manga ended on chapter 57. The WN is currently on chapter 187. I couldn't find an English translation, so here's a link to the WN in Japanese in case anyone is interested...
She's a girl ghost named Tamiko. I think you guys got confused by the fact that the artist hasn't been bothering drawing her long hair consistently since like chapter 2, so she's been looking kind of boyish.