So the girl who turned him was never important ?

I also had this doubt, I thought her would be the solution since her started with this

I thought so too

I thought that the whole point of the story was for Shion to realize being a player can and did hurt the girls he hooked up with and than took off, and once he decides to not do that anymore he turns back into a guy on his own. Guess it was fuck I don't know what.

I guess it was less about learning his lesson and more to do with having an excuse to start sleeping with his best friend x)