Rick Astley
Rick Astley is a singer in real life, who has not appeared in the remix world proper, but has appeared in non-canon remix tree content. He is known for his song "Never Gonna Give You Up", which is often the subject of "rick-rolling", a prank wherein people unexpectedly spring the song on others.
There have been multiple rick-rolls in the remix tree: the title of dQw4w9WgXcQ, the contents of Super Half-Life 4++: Special Edition (Now featuring Expert mode), and the ending of An April Fools Hope.
History
Majora's Mask sung Never Gonna Give You Up backwards while superimposed over an image of him.[1]
In a non-canon timeline, Josef Mengele turned out to have been him in disguise all along.[2]