Someone who won't take any of your crap. Someone you may fight with, who may hurt you, but who's there anyway. Homicide deals honestly with partnerships, often acknowledging that partners aren't necessarily friends, that the relationships may not last once one member of the team leaves the department. Partners may grow closer or drift apart as time passes. The partnerships are not idealized.
Partners shift and change, too.
Sometimes new partnerships work, sometimes they don't. They grow, they evolve. Partners spend so much time together, they know each other well. They don't choose their partner, usually, they're thrown together. Sometimes there's magic in these pairings.