Triangle Man is an olive green triangle
Marshmallow People He appears riding on a flying surfboard and jumping off of it and claiming that he "just surfed around the world" and then Pink Marshmallow starts to stab him for no explained reason and then White Marshmallow and Pink Marshmallow kidnap Triangle Man and Triangle Lady and Triangle Man starts claiming that he could feel his "Triangular awesome-ness leaking out of his face" and after that Triangle Lady eats part of Triangle Man.
Marshmallow People 2 He is seen juggling balls until White Marshmallow and Pink Marshmallow start kicking him until Triangle Lady shows up and the two marshmallows start kicking Triangle Lady. He is seen bruised up when the two marshmallows kicked him earlier and White Marshmallow and Pink Marshmallow visit Triangle Man again to "give everything back" and he says "Thats your problem, i'm outta here" and Triangle man floats up into the sky while changing colors and turns into a tree.
Marshmallow People 3 The two marshmallows go over to Triangle Man's house to eat his face but Triangle Lady beats them to it and Triangle Lady asks White Marshmallow and Pink Marshmallow if they would like to eat her face instead and White Marshmallow replies "Maybe" then the two marshmallows and Triangle Lady and Triangle Man ride on the otter with White Marshmallow wondering what they all should do and White Marshmallow says "Well we need at least one face left" and Triangle Man claims White Marshmallow to be selfish and then White Marshmallow says "Your being selfish, i didn't get any of your face" but just then a rainbow like beam hits the otter causing Triangle Man to lose balance and fall off the otter then Triangle Lady suggests that they should get Triangle Man back but White Marshmallow says to leave him because "He's being an asshole"