Umm

In the MC's childhood memory, there're three children: himself (the dark-haired boy), a kid getting bullied and a bully. It turns out that the bully is actually the girl/ghost Mi- Chan (who bullied him to get his attention coz she liked him too), while the bullied kid is actually Hikari (or was he Hikaru?), who is the one the ghost is visiting. So she's kinda leaving him in the MC's care, since he always looked after him when she bullied him, and because Hikari also has some feelings for him.

Hmmm that does make a lot of sense now