He is a snow monster created by Elsa to serve as her bodyguard.
Kingdom Hearts III
Marshmallow is created by Elsa's strong desire to be alone, taking the form of a brutish snowman. He then promptly evicts Anna, Kristoff and Olaf from Elsa's ice palace, but turns aggressive when Anna throws a snowball at him. However, before he can give chase to the trio, Sora, Donald, and Goofy block the giant and lead him to another area, where they use the area's trees to wallop him. Although they are able to hold their own, Marshmallow manages to corner them on a rocky outcropping, which he breaks off causing them to fall down the mountain.
Later, Marshmallow pursues Hans down the mountain, trying to rescue the kidnapped Elsa, and runs into Sora and his friends. When Sora explains that they are trying to help Elsa as well, Marshmallow helps them down the mountain, shielding them from the growing storm.
Once Hans fails to kill Elsa, the darkness in his heart births the Sköll, which transfers the quartet to a dark area. During the battle, Marshmallow helps repel the Heartless's most powerful attack[?] alongside Sora.
Sometime after Master Xehanort's defeat, Marshmallow catches Olaf, whose parts ended up separated before fixing himself.
Marshmallow is a large golem made of snow and ice, with the majority of its body being composed of snow. Icy protrusions function as Marshmallow's fingers. Its eyes and mouth consist of gaps in the snow that makes up its head.
He is shown to be a loyal and prudent bodyguard who keeps every intruder outside of Elsa's castle. However, is is shown to be relentless at the slight sign of provocation. When Elsa is taken away, Marshmallow is shown to be helpless and desperate whenever Elsa is not around.
Sometime after Master Xehanort's defeat, he is shown picking up Olaf's parts while smiling, showing a playful side.
Marshmallow first appeared in the 2013 Disney film Frozen, voiced by Paul Briggs. In the film, Elsa first creates him to shut out Anna, Kristoff, and Olaf when they come to convince her to return to Arendelle. He pursues the trio off of the North Mountain, and later attempts to protect Elsa's castle when Anna's fiancé Hans arrives with an army to capture Elsa, only to seemingly fall to his death when Hans cuts off one of Marshmallow's legs. At some point, Marshmallow returns to Elsa's castle and reclaims her tiara.