Nick
Nick is his older half-brother. The relationship between them is difficult, as Nick used to bully him. But even if he doesn't show it he does care about him. Over the series they've become closer and closer as Nick has been reconciling with Neil.
Manhua
In the manhua Neil makes it clear that he dislikes Nick, often complaining about him and doing mean stuff towards him to seek revenge for all the days Nick used to 'bully' him.
Ira
Ira and Neil know each other from online games, ending good internet friends, enough at least for the demon to go to Ira when he decided to leave Hell.
Lily
Neil started to have a crush on Lily when she explained him that what others say doesn't matter at the moment he does what he think is right. The two relate to each other and are great friends, even if the demon tries to show his love to her.