The first killer is Akari, who killed the first three victims with the signature of the popular girl who lost a relative in the war. The motive was she was inspired by [The Smiles of Heaven].
The second killer is Winter, with the signature of killing girls who take care of their brother. He is a fan of Seguko... and so he is a fan of Masahiro Junsuke, because they are the same person! Seguko speaks of Junsuke like he is a different person because he feels detached from him, as if when he writes as Junsuke he's on a whole different level, and not himself.
Winter followed [The Smiles of Heaven] as well, seeing if Naito would smile in the end, after all that happened (that's the reason for the mockery), as the protagonist of the novel did.