Tokita Jun

The protagonist. His father remarries and he finds himself with a new mother and 3 step sisters. Despite how she looks somehow Tsubame is older than he is. After his father has been married for awhile and he's settled into his new family life he discovers that his step mother is in fact a succubus. It turns out all his sisters are succubi too.

They need to have sex frequently in order to become adults but he doesn't want to have sex with them because he's not a lolicon.