Akuma-kun no Kanojo

Shiragiku Amano's dream is to become an "angel in white" like her mother. She can't just leave injured or bad people alone! One day, she gets caught up in a fight with the delinquent group BLAX, and is mistaken for a member of BLAX by the opposing group. In the midst of all this, she pretends to be the girlfriend of BLAX member Kuroya in order to be protected from their enemies. In addition to Kuroya, BLAX also has the womanizer Akao and the slightly mysterious Aonami.

(Source: Ciao Plus, translated)