In the Ultimate comics, Spider-Man typically weighs around 167 pounds, as stated in the comics.
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Yes, Ymir has special dialog with certain characters in Marvel Ultimate Alliance, such as Thor. Ymir is a Frost Giant and a frequent antagonist of Thor in the Marvel Comics, so their interactions in the game reflect this rivalry.
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Thor's real name is Thor Odinson. He is a character from Norse mythology and Marvel Comics, known as the Norse god of thunder.