Then let's use the story's internal psychological ideas here.
There are two signatures to these killings. We were told by a presumably reliable character that signatures do not change.
Signature 1: A girl is killed who lost a close relative in the war. She is cheerful and popular.
Signature 2: A girl is killed who did not lose any relatives in the war. Whether she is popular is irrelevant. However, a taunting message is left on the body for the detective.
Two signatures mean two killers by the story's logic.
The first killer was Akari Junchiro, who was a huge fan of the novelist Masahiro Junsuke. She envied the popular girls and wanted to be the 'hero', or protagonist. She modeled the killings after the book [The Smiles of Heaven].
The second killer was Entror. He took advantage of the fact that he had a clear alibi for the first set of killings, and imitated their MO to begin a cat-and-mouse game with the detectives, figuring that he was above suspicion. He heard Winter use the term "Mr. Detective," and put that in the notes as a blind. His goal is to prove his superiority.
Edit: Alternatively, I'm just overthinking this and the second killer is Winter, not Entror, who was familiar enough with Seguko's novels to want to match wits with him, openly referred to 'Mister Detective', and planned on revealing that he was gambling with Entror when one of the first murders happened, revealing an alibi that would throw the investigation.
Either way, the love of risks and gambles was a major part of Killer Two's motivation -- the chance to play a game for major stakes.