Ken Kurosawa

He is one of Fumino's classmates and has a crush on her. Although Ken doesn't know that Kazuma and Fumino are married, he originally sees them together hugging, when Kazuma was telling Fumino that her life was more important than their marriage ring.

He later walks in on them right when they were about to kiss after Kazuma had whisked Fumino away into a classroom to watch the festival's fireworks together.

Source: Wikipedia