The doppelgänger is a being that can transform into any person it chooses. It is said that whosoever meets it will die within a few days, and much folklore stems from this creature. They had also been said to 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 most other appearances, Doppelgänger actually mimics the protagonist's appearance, presenting itself as a grinning, shadowy copy of the hero.
Known as Kageboushi (カゲボウシ, lit. shadow figure), 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.
Doppelgänger is the antagonist of episode 21. He's a very powerful demon who was sealed away a long ago and freed by Lucifer. Doppelgänger pretends to be Setsuna Kai in order to eliminate the Devil Children, but ends up developing a crush on Mirai Kaname and instead tries to eliminate Cool and Veil so they can be alone together. His romantic advances make Mirai suspicious and ends up being the key to finding out which Setsuna is the real one. Doppelgänger is a very skilled opponent who can also copy the spells of those whose appearance he imitates. In the end, Doppelgänger proves himself to be such a dangerous enemy that, instead of being defeated, he's sealed by Franken using the same ritual that trapped him before.
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.