Responses
That is very interesting theory,but I think Summer also refers to when Yoshiki found out that Hikaru was no more and confronted the monster Hikaru,aka the beginning.
Hikaru-moster said he couldn't heal so well if he had a missing half and he was still recovering when he gave Yoshiki the half so maybe Hikaru-moster died in this summer
My other theory is that they became one in some way and are living together in another place
I honestly think it's a good ending if both of them goes to the other side. Both Yoshiki and Monster Hikaru looked so happy, and if they can immortalize that happiness and love they feel for each other on the other side, than that makes me happy.
If the real Hikaru did not die at summer, could the summer that Hikaru died refer to this current summer? Where Monster Hikaru dies?