It can transform into any person it sees. It is said that whomsoever meets it will die within a few days, and much folklore stems from this creature. They may also become physical incarnations of psychological ailments.
"A phantom copy of a living being. The name comes from the German phrase "double goer" Doppelgangers are a sign of bad luck. Often, others see your doppleganger from afar, but it is said you may also see your own doppleganger right before you die."
—Shin Megami Tensei: Strange Journey Compendium
Unlike other appearances, Doppelgänger actually mimics the protagonist's appearance, presenting itself as a grinning, shadowy copy of the hero.
Devil Summoner 2: Raidou Kuzunoha vs. King AbaddonEdit
Known as Kageboushi (カゲボウシ), it is the last Fiend that can be randomly faced in the game. No matter how many times he is defeated Kageboushi will appear again should a Fiend battle happen. He comes in three types: Sword, Spear, and Axe. Each has its own resistances, weaknesses, and skills. Kageboushi drops the Aletia needed for a Case File and Sword Alchemy, along with a decent amount of money and EXP when beaten. However, Kageboushi also randomly runs from battle. When defeated he will mockingly wish Raidou the best of luck, laughing as he dies.
Doppelganger is both a demon and a BattleNet opponent, taking the form of the Protagonist and facing them with the same demons in S Class. Lucifer also gives a Doppelganger after beating him in BattleNet.
Doppel is a B-Rank Battlenet Opponent who takes the form of each versions Protagonist. After beating Doppel several times it gives a unique version of the demon Leogard named Powerleo. Beating him again earns the player a Fenrir.
Doppelgangers usually are resistant to physical skills of all kinds, usually being capable of outright reflecting all physicals attacks. This could be attributed to their own nature as shadow copies of the protagonist, where all the harm done to them goes back to the protagonist, for being one and the same.