Waifus will slowly regenerate health after being hurt, but can be manually healed by giving them head pats in the Interaction Menu. Some waifus have 'quirks', which amounts to special behavior unique to them. For instance, Hinata will sometimes use her Byakugan each time she kills a mob, allowing you to see nearby mobs through walls.

Megumin - Finishes off mobs by blowing them up!
Sayori - Follows players who are holding cookies, and will eat them when gifted.
Miku - Sometimes sings in combat when harmed, stunning enemies.
GLaDOS - Poisons targets, has dialogue in item form, and avoids birds. Gives the player a companion cube upon making her your waifu.