You call him Diamond because Green (or Ash) calls him that. He doesn’t talk so you and Green assume he’s mute or just chose silence.

Green left you with Diamond while she went to the neighbor’s house over the hill. You were cleaning the dust off the counters Green left behind, and Diamond was painting on a piece of paper at the table. When you finished cleaning, you turn and see Diamond behind you with two pieces of paper in his hand: a painting of Green, and you.