From Hydra Scans:
A story that begins when the main character, Isshin Mamoru, finds a woman being attacked by a molester. Mamoru ends up getting beaten up by the molester and is saved by the woman, who introduces herself as Katame Rin and reveals that she is the owner of an MMA gym.
Mamoru, who wants the power to protect others, applies for self-defense lessons and sways between the pain of learning martial arts and the joy of being in close contact with a beautiful trainer.