Balancing the party? (Dungeons and Dragons thread)

Yes, I know this should go under "Dungeons and Dragons", but nobody ever looks there.
I need help finding a class for my brother to play in my campaign. Right now, I have two players, a wizard and a tank fighter, as well as a fighter NPC. What class should he play to keep things balanced? A healer might be needed, or maybe a bard, but I'm not sure. Suggestions?

Nevermind he decided to be an Forest Gnome Arcane Trickster who rides on his familiar (it's an owl so it works)