irregular_(chaban_capriccio)
An Allen Avadonia look-alike first appearing in Chaban Capriccio (vocaloid), who is implied to be the same individual as the main character of Re birthday (vocaloid). His true identity has flip-flopped over time via word of god statements as to whether or not he is, in fact, Allen Avadonia or someone else.
As of 2020, with the new upload of Re_birthday combined with the master of the heavenly yard (vocaloid) novelization, it seems to be that the character in Re_birthday is, in fact, Allen Avadonia, while the character featured in Chaban Capriccio (and mentioned in master of the court (vocaloid)) is actually Adam (master of the heavenly yard).