from what i’m aware of, the person in the floor, aka the person who was calling out tsukasa, made a deal with tsukasa being that if they made amane healthy (amane was said to not have a chance of survival) tsukasa would have to sacrifice himself which he did do in the end. then when the person in the floor fulfilled his wish, tsukasa possibly gave up his body (now being dead since the age of 4) and the spirit took it over so since the age of 4, amane and his family grew up with a different tsukasa who wasn’t the real one. however there’s also the possibility the tsukasa who killed mitsuba was a manifestation of amane’s guilt for killing “tsukasa”.
OHHH okay i understand thank you
WAIT WHAT!? THE WHO IS THIS TSUKASA WE KNIW WHO KULLED MITSUBA ETC? WTF IM CONFUSED