From the Super Mario Wiki
Letters are objects that appear in Super Mario Odyssey. They are made up of a red M, a green A, a yellow R, a blue I, and a pink O. They appear post-game in the Metro Kingdom where Mario must capture each of the Letters and move them into slots to spell "MARIO", after which Mario will be given a Power Moon. The player can press and hold + to dash and shake the to dash faster while in this form.
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