Narrative secrets ahead.
Marshmallow is a character in the 2013 animated film Frozen. Probably he is the personal bodyguard of Elsa the Snow Queen, and acts as a security guard to the North Mountain. He is voiced by Paul Briggs.
Just as Olaf represents Elsa's childlike persona, Marshmallow appears to represent Elsa's powerful desire to be left alone. Because of this, he is extremely aggressive and territorial to intruders or others who try to enter the castle except for Elsa, whom he was made to protect. He is incredibly devoted to her in this role and continued this even when he was injured. However, at the end of the film, when returning to the ice palace, he is shown smiling and letting out a happy sigh after placing Elsa's old tiara on his head. This possibly hints that while he represents her desire to be alone, he could also contain some aspects of her innocence.
Despite being a highly competent minion for the most part, Marshmallow still has some bumbling edges as seen when chasing Kristoff and Anna, who were able to outsmart the snow beast, having him smacked by a tree.
Marshmallow may also be a bit hotheaded as he became angered and chased both Anna and Kristoff just because Anna threw a snowball at him. Yes, he is Hans's enemy, but this is not mean he is good or neutral.
Marshmallow is a hulking behemoth of a snow man, whose joints are comprised of ice rather than sticks, unlike Olaf. His fingers are made from rounded icicles and are really sharp like knives, his elbows and knees having sheets of ice on them like knee and elbow pads.
When angered, it is shown that Marshmallow's eye sockets light up demonically and ice spikes will protrude from multiple parts of his body. It is shown that he gains ice spikes on his back, somewhat like the quills of a porcupine or hedgehog, as well as ice growing along his arms. His ice fingers sharpen to become talons, and fangs will grow along his jaw line. This helps make him a more formidable fighter, as well as making him appear more threatening.
Despite being made from snow and ice, he seems to have no real control over it. However, Marshmallow has immense physical strength, which he uses as his primary weapon. He can grow ice from himself, however, in the form of weapons such as spikes, claws, and teeth to compose armor. He also has shown the ability to breathe blizzards of snow from his mouth. When facing one or two or three opponents, his immense strength can give him the upper hand. However, he does have slight difficulty taking on large groups of opponents at one time, due to their strength in numbers and his lack of agility.
Like Elsa, his abilities are (usually) controlled by his emotions. When peaceful, Marshmallow's appearance is smooth and warm, much like Olaf, though still hulking due to his size, but when angered, the spikes and shards are unleashed, and the more his fury, the more deadly ice shards consume his body, making him all the more dangerous and powerful.
After Anna arrives in Elsa's ice palace, the princess tries to convince her sister to return home, to Arendelle. However, Elsa refuses, though Anna remains persistent even after Elsa's frequent requests to leave and unintentional freezing of her sisters heart. With no other choice, Elsa creates Marshmallow as a personal bodyguard to send Anna, her companion Kristoff, and Elsa's first enchanted snowman, Olaf, out of her home, to never return. After Marshmallow kicks them out however, he upsets Anna by throwing Olaf onto a snowy boulder. As revenge, Anna balls up a snowball and tries to throw it at the giant beast, but Kristoff, knowing it's a bad idea, keeps Anna from throwing the snowball, telling her to let the giant snowman be. Anna tells him she's calm and seemingly softens, but the moment Kristoff turns his back, she throws the snowball at the giant snow monster. Though it leaves literally no damage whatsoever, the lack of respect is enough to infuriate Marshmallow, causing him to chase Anna, Kristoff and Olaf down the Northern Mountain.
Marshmallow manages to corner them at the edge of a cliff, though Kristoff immediately begins digging a snow anchor, using a rope to safely guide himself and Anna down the mountain to safety. However, Marshmallow catches up to them, though Olaf tries to stop him. Annoyed, Marshmallow kicks Olaf over the cliff, and continues his chase for Anna and Kristoff, pulling them up to his face by the rope, and ordering them once more not to return, just before Anna grabs Kristoff's knife, cuts the rope, then sends the duo plummeting down, though they survive. With his mission to drive them away complete, Marshmallow returns to the ice palace.
Later, Marshmallow stakes himself outside of Elsa's castle, disguising himself as snowy boulders in case more unwanted guests were to arrive. Eventually, they do, in the form of the conniving Prince Hans and his army, set on finding Anna and capturing Elsa. The moment they come close enough, Marshmallow reveals himself and jumps right into battle. The soldiers immediately attack the beast with their arrows, enraging Marshmallow and causing his ultimate form to be unleashed. Marshmallow is able to hold most of the guards off, though, Hans proves to be a fierce warrior himself, avoiding each of Marshmallow's attacks and eventually using his sword to slice the snow monster's left leg off, causing him to lose balance and begin tumbling over to a large gorge. With Marshmallow wounded, Hans begins heading inside Elsa's castle, but Marshmallow doesn't give up, giving one last swing in attempt to drag Hans down with him. Unfortunately, he fails, and the giant snow beast plunges down into the chasm below, apparently to his death.
After the credits, it's revealed that Marshmallow had survived the ordeal, and is seen limping back into the castle with a new-made left leg where all he finds is Elsa's crown. The monster looks around for a moment and smiles. The monster, pleasing his inner prince, happily crowns himself ruler of the castle, causing his ice spikes to disappear as he gives a happy sigh, happily continuing his life in the North Mountains as its ruler. That however do not mean now he is good.