If the LaCroix/Hatfield clan were the Addams Family, Nick would totally be Uncle Fester. After all, when Tia first makes his acquaintance in the days before Sam's birth, she discovers him living on the Oregon coastline in an isolated, one-room cabin that hasn't been cleaned in months and doesn't have a bathroom. There's also the whole thing about him being a necromancer. In any case, all of it adds up to Nick being kind of a weird guy. Of course, his reason for being isolated is to avoid Douglas coming after his powers, so maybe it's not so weird after all.
The main thing to know about Nick is that he and Kevin don't get along very well. When Tia asks if he'd like to make contact with her husband, Nick's response is a definite no. "I was born," he explains, "and Kevin's never gotten over it" (12.25). We can't say for sure, but it sounds like Kevin's hated his brother his entire life, probably because Nick is a glimpse of who he could have become had the genetic lottery gone the other way.
Meanwhile, Nick's most defining trait is his need to dispense information about the family's necromancy leanings. When Kevin's first daughter with Elaine is born, for instance, he shows up at the hospital saying that Lilly "might need a guide. She might be dangerous" (16.27). Kevin apparently responded to this attempt to warn Elaine of the necromancy gene by punching him. His interest in both Sam and Lilly's wellbeing as recipients of the ability reveals a stark contrast between how Nick and his brother respond to it.