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 took over Astley's body and sung Never Gonna Give You Up backwards..[1]
Non-canon
In a non-canon timeline, Josef Mengele turned out to have been Rick Astley in disguise all along.[2]