Sakurazuka Tsukumo

He is the onii-chan from Natsuki's timeline ( when she is an adult and a virgin ) she met in her childhood.

He is an alternative version of the protagonist, his history is a quite different. He also lived the same things as in the first chapter but with variations :
For exemple, he don't know the existence of Mimi because she never existed in his timeline and the events he lived such the rescue of Mimi becomes - for him - the rescue of Akeno ( who takes her place in his story ).

When he returns in the past for saving his sister from her injury in 1999 and when he comes back in the present timeline, his sister was with another "himself", the one from the timeline of the Natsuki of 1999 who never has to seek to travel back in time in order to save his sister.

His return to the present makes him exist in duplicate. And that's not the only thing that has changed.

Some people he knew are completely different, their lives are different. His brain contains two versions of his memories, those of events that have been known since the beginning of the game and new ones that are parallel.
Tsukumo's mission is far from over and he decides to go back in time with "Doc" in 1996 ...

Being the same Tsukumo that the player controls since the beginning of the game, he keeps all the attributes of his counterpart, just his way of dressing changes.
He's the protagonist of the third chapter only.