Isaac is William's closest and only friend at school. The two are close, even though Isaac annoys William sometimes. Isaac is also an exceedingly important person to William (demonstrated when Michael inquired as to what William felt was most important to him).
Since Dantalion spends most of his time with William, the two appear to be friends.
Isaac and Sytry have not interacted past their first meeting. Though, Isaac may be friends with Sytry too.
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 and Isaac was interested when he found out Kevin was an angel. Kevin gets along with Isaac too, but won't hesitate to erase Isaac's knowledge of him being an angel if the need arises.
During the last chapters of the manga, Isaac surprisingly becomes very fond of Michael and takes care of him when he is in his weakest state. Michael also protects Isaac when they are attacked by Sytry and his legions of Angels in William's manor. Isaac is, in fact, one of the few individuals for which Michael has shown some real concern.