Bokusatsu Tenshi Dokuro-chan

Sakura Kusakabe, a mere junior high student, has committed a horrible crime. Or, rather, he will commit a crime—20 years into the future. The crime in question? He will prevent all women from developing past the age of 12, with immortality as an unintended side effect. As a result, the angel Dokuro Mitsukai is sent from the future as an assassin, armed with with a magical spiked bat named Excalibolg. However, rather than kill him, Dokuro wants to take a different approach. She resolves to be a constant nuisance to Sakura, in an attempt to prevent him from focusing on anything long enough to create his criminal technology.