Unknown person, unknown Bakugan or unknown alien.
In BMK series:
Unknown New Battle Brawler
Bakugan Attribute: Darkus
Komand'r Kar'lloz appears in the episodes "Grand Prix", "The Invasion of Kha'rall" and "The Dark Lizard Surge" in BMK series as the unknown creature, but he presents himself in the episode "The Dark Lizard Surge".
There are more unknown characters in BMK and in another series, to be announced.