Isaac is William's closest,and only, friend at school. The two are close, even though Isaac annoys William sometimes.
The two appear to be friends.
Isaac and Sytry have not interacted past their first meeting. Though, Isaac may be friends with Sytry.
As the head boy, Isaac respects Camio. As a demon, Isaac is fascinated by Camio. Camio has not shown any ill will towards Isaac, so the two may be friends.
The two get along. Isaac was interested when he found out Kevin was an angel. Kevin gets along with Isaac, but won't hesitate to erase Isaac's knowledge of him being an angel if the need arises.