Marshmallow is one of the main characters in the Annoying Orange. He is known as the cutest character in the series, and his best friend is Midget Apple. He first appeared in Annoying Saw 2, then eventually became a main character. Marsh may be genderless, as this is unknown.
Usually, Marshmallow is not angry or frustrated but, in the episode Best Fiends Forever he got very angry because Grapefruit burns a picture of Marshmallow's favourite unicorn, Princess Butterfly Kiss. He then proceeds (offscreen) to attack Grapefruit which caused the house to explode. This side of his personality would become more common in later episodes. He often laughs and has the same sense of humor of Orange. He has a funny laugh that goes, "Hehehehehe!" and "Yay!". Orange and he seem to be close friends. Orange doesn't annoy Marshmallow at all. Marshmallow also has a high-pitched, squeaky voice and has the personality of a child.
Marshmallow is a cheery, bubbly little soul; he is always happy but will instantly switch to the emotion that suits the current problem, it is shown that when he sneezes, magic, sparkling glitters spray out of his non-existent nose. He likes "Rainbows, puppies and kittens and flowers and Rainbows and clouds and bunnies and Rainbows" (as he said in the Annoying Orange meets Charlie The Unicorn Spoof). He is never sad, angry or unhappy unless the situation justifies it. He also has a profession in writing song lyrics, despite the overuse of rainbows in them.
Orange was seen playing Battleship with him on Annoying Saw 2.
In Best Fiends Forever, it is seen that Marshmallow can be destructive when pushed to the limit, and apparently has enough power to make the whole the kitchen explode and propel Grapefruit out of the house.
His favorite song is Space Unicorn by Parry Grip. 
Due to his feminine nature, fans have questioned several times if he is a boy or a girl.