kego nagami

Ahhhh now yes! Keigo Magami (Magami Keigo) from Witch Watch!

The correct name is Keigo, not Kengo - that's why I couldn't find it.

He's the werewolf of the house.

Keigo's personality:

• In human form: he is super quiet, introverted, and has low self-esteem. He likes being alone, enjoys otaku/hipster stuff (cult films, indie games, obscure manga). He constantly thinks of himself as a poser failure and gets depressed easily. He doesn't have much self-confidence.
  But he's very loyal. When he joins the Otogi family (Nico, Morihito, Kanshi), he starts to change and begins to enjoy being with them, considering them family.

• When he sees a crescent moon (even if it's a drawing) he turns into a WOLF:
  Then he becomes a different person: loud, violent, overconfident, muscular, and cocky. He's like Jekyll & Hyde. Wolf is his repressed wild side.

Fun fact: he was a professional figure skater as a child, but stopped after his mother had an accident.