Game ni Natta yo! Dokuro-chan ~Kenkou Shindan Daisakusen~

Bokusatsu Tenshi Dokuro-chan tells the story of 13-year-old junior high schooler Kusakabe Sakura, who twenty years in the future develops a technology that causes all women to stop physically aging after they reach twelve years old in an attempt to create a "Pedophile's World". However, this act accidentally creates immortality amongst humans, thus offending God. Mitsukai Dokuro, a member of an order of assassin angels that are called Rurutie, has been sent from the future to kill him. Believing that Sakura can be redeemed, Dokuro decides instead to keep Sakura so occupied and distracted that he can never develop the immortality technology.

Due to her impulsive nature and super-human strength however, she frequently kills him with her gigantic spiked kanabou (club), Excalibolg, on a moment's impulse, but returns him to life moments later with angelic power, typically reciting her personal chant "Pipiru-piru-piru-pipiru-pi" (which has no meaning at all and is just a habit of hers).

Mihashigo Sabato, another assassin of the Rurutie order who uses her feminine wiles along with her electric baton to do her job, is dispatched to complete Dokuro's original mission: Sakura's assassination.

[Edited from Wikipedia]

The game features Sakura trying to establish a relationship between him and any of the girls in his environment.