Treek can be either a Tank or a Healer. As a healer she (yes, she!) is at least on par with other healing companions, but she really shines as a tank compared to all other tank comps. She uses taunts intelligently and has better threat generation in general.
She has affection conversations, but no associated quests. The conversations differ based on your faction. She does not unlock anything for crew skills, sadly, but does have a +1 Crit bonus.