If you're looking for manga similar to Grisaia: Phantom Trigger, you might like these titles.
Kikuno Kei, nicknamed "Kay", is a normal university student and a part-time hitwoman with John Wick like skills.
A spate of mafia assasinations occurs in America, the work of the mysterious organization named Inferno. "Phantom" is their most strongest and capable assassin, however a lone tourist witnesses Phantom's latest act of murder. He unwittingly exposes the top assassin's true identity, a young girl named Ein. Witnesses are usually killed, but the man is captured and taken to be brainwashed into "Zwei", Inferno's newest assassin, with no memory of who he is. From this point on, Zwei is drawn into a world of conspiracy, deceit, and violence.
High-school student Jinpachi Kogure is a member of the manga club and is no good with women. Yumika Testuyoshi is a troubled female high-school student who's always skipping school and searching for a girl she doesn't hate. However, these two are in reality young masters of the art of killing people: "Young Guns"!
Imari Matoba looks to be an ordinary, physically-weak high-school student. But this quiet, nondescript girl is actually an assassin - a top gunfighter with unlimited potential who can take down several armed men at once...
When you're the infamous prodigy hitman known only as "Fable," many things come easy. Being a normal person, however, isn't one of them. In fact, being told that he can't kill anyone for a while may just be the hardest job Fable's ever taken...
High-school student Jinpachi Kogure is a member of the manga club and is no good with women. Yumika Testuyoshi is a troubled female high-school student who's always skipping school and searching for a girl she doesn't hate. However, these two are in reality young masters of the art of killing people: "Young Guns"!
Taro Sakamoto was once a legendary hit man considered the greatest of all time. Bad guys feared him! Assassins revered him! But then one day he quit, got married, and had a baby. He’s now living the quiet life as the owner of a neighborhood store, but how long can Sakamoto enjoy his days of retirement before his past catches up to him?!