The Doppelganger is one of the rarest and hardest cards to achieve in both Lost Kingdoms and lost Kingdoms II.
It is never found in the wild, the only way to get it is to transform the God of Destruction or one of the four Sacred Beasts into it for 9999 Experience.
Unique to this monster is that it's card art changed between games. In Lost Kingdoms, the Doppelganger card depicts Kata whereas in the second game Tara is shown instead. This is meant to reflect that it is copying the appearance of the summoner (which is exactly what a Doppelganger is; an apparition or double of a living person) though this does not occur if you change your character in the 2P VS Mode.
Doppelganger, once summoned, will wander the level until it bumps into another character. Be it friend, foe or something else, the Doppelganger will instantly kill that enemy, even if said enemy is a Boss. This can also affect friendly monsters and the player, so Dopppelganger has the ability to back fire if it is not kept track of.