Anonymous is a Miiverse user claiming to be the Miiverse account of the real-life hacker group Anonymous. Despite having posted only twice, he (or they) are infamous among the YouTube and Wii Fit U communities and has 470 Yeahs plus 238 comments and counting on their first post, which happens to be the slogan of Anonymous in real life. This account is likely a troll, and judging by the reactions of the aforementioned YouTube community, a very effective one at that.
Anonymous's Mii is hidden, likely to add to the feel that the creator of the account (who remains unknown at this point) intended when creating the account.