Doppelganger

A Doppelgänger is a creature who takes the form of another, usually the one it is attacking.

Castlevania: Symphony of the Night
The Doppelgänger (spelled "Doppleganger") boss has the form of Alucard and appears twice.

Doppleganger10 is the second boss of the game. (Third counting with Dracula from prologue). Its uses the dagger, can turn into mist and bat before Alucard can. Doppleganger10 can also use a shortsword.

Doppleganger40 is a stronger Doppelganger with new moves. He can use the Bouncing Diamond subweapon, Gravity Boots, Leapstone, as well as all of Doppelganger10's abilities. In addition, Doppelganger40 will use whatever weapon Alucard is using, allowing for some strategy (such as using Terminus Est to have him do poison damage, while having the Topaz Circlet equipped, so the doppelganger's attacks heal you.)

Castlevania: Lament of Innocence
The doppelganger boss is fought in two areas in the game. The first fight is in the Underground Waterway, where his weapon is a mockery of the Vampire Killer (the copied whip glows red) and has a blue body. The second fight is before the second floor of The Pagoda of the Misty Moon. This time it glows red, has more HP and attacks with its fists. In Joachim mode, the Doppelganger is called "Leon Fake", and the Joachim boss is called "Doppelganger", but this name change does not affect gameplay at all.

Castlevania: Dawn of Sorrow
The player does not physically fight the Doppelganger in this game but there is a Doppelganger soul that allows the Soma to switch between two sets of weapons, souls and armor quickly. Celia Fortner kills a Doppelganger in the form of Mina Hakuba in an attempt to turn Soma Cruz into the new Dark Lord. Dmitrii Blinov later uses the Doppelganger's body to resurrect himself.

Castlevania: Curse of Darkness
In Trevor Mode, the Trevor boss is replaced by the Trevor Doppleganger. He is exactly like Trevor (besides glowing dark blue and being weak to light) and has the same moves. He has no entry in the bestiary. In the second fight he gets more agressive, so use caution while fighting him. You only face him twice.

Castlevania: Portrait of Ruin
Can be found in the deepest level of Nest of Evil. To make the final battle with the Doppelganger easier, just unequip everything off of BOTH characters before entering into the room (this includes skills/spells). After you enter the room, put your stuff back on and have fun whacking an unarmed opponent. Of course, if you want a challenge, do the opposite and equip your best stuff before entering. Generally whichever character is stronger and better equipped is the one the Doppelganger will become. After you kill him you can acquire the final Dual Crush skill The Greatest Five.