Doppelgangers are strange beings that are able to take on the shapes of those they encounter. In its natural form, the creature looks more or less humanoid, but slender and frail, with gangly limbs and half-formed features. The flesh is pale and hairless. Its large, bulging eyes are yellow with slitted pupils. A doppelganger?s appearance is deceiving even when it?s in its true form. A doppelganger is hardy, with a natural agility not in keeping with its frail appearance.
Doppelgangers make excellent use of their natural mimicry to stage ambushes, bait traps, and infiltrate humanoid society. Although not usually evil, they are interested only in themselves and regard all others as playthings to be manipulated and deceived.
It is natural form a doppelganger is about 5-1/2 feet tall and weighs about 150 pounds.

When in its natural form, a doppelganger strikes with its powerful fists. In the shape of a warrior or some other armed person, it attacks with whatever weapon is appropriate. In such cases, it uses its detect thoughts ability to employ the same tactics and strategies as the person it is impersonating.
Detect Thoughts (Su): A doppelganger can continuously use detect thoughts as the spell (caster level 18th; Will DC 13 negates). It can suppress or resume this ability as a free action. The save DC is Charisma-based.
Change Shape (Su): A doppelganger can assume the shape of any Small or Medium humanoid. In humanoid form, the doppelganger loses its natural attacks. A doppelganger can remain in its humanoid form until it chooses to assume a new one. A change in form cannot be dispelled, but a doppelganger reverts to its natural form when killed. A true seeing spell or ability reveals its natural form.
Skills: A doppelganger has a +4 racial bonus on Bluff and Disguise checks.
*When using its change shape ability, a doppelganger gets an additional +10 circumstance bonus on Disguise checks. If it can read an opponent?s mind, it gets a further +4 circumstance bonus on Bluff and Disguise checks.
SEE WIKIPEDIA ENTRY: Doppelganger

Doppelganger characters possess the following racial traits.

• +2 Strength, +2, Dexterity, +2 Consitution, +2 Intelligence, +4 Wisdom, +2 Charisma.
  
• Medium size.
  
• A doppelganger?s base land speed is 30 feet.
  
• Darkvision: Doppelgangers can see in the dark up to 60 feet.
  
• Racial Hit Dice: A doppelganger begins with four levels of monstrous humanoid, which provide 4d8 Hit Dice, a base attack bonus of +4, and base saving throw bonuses of Fort +1, Ref +4, and Will +4.
  
• Racial Skills: A doppelganger?s monstrous humanoid levels give it skill points equal to 7 x (2 + Intmodifier). Its class skills are Bluff, Diplomacy, Disguise, Intimidate, Listen, Sense Motive, and Spot.
  
• Racial Feats: A doppelganger?s monstrous humanoid levels give it two feats.
  
• +4 natural armor bonus.
  
• +4 racial bonus on Bluff and Disguise checks. When using its change shape ability, a doppelganger gets an additional +10 circumstance bonus on Disguise checks. If it can read an opponent?s mind, it gets a further +4 circumstance bonus on Bluff and Disguise checks.
  
• Special Attacks (see above): Detect thoughts.
  
• Special Qualities (see above): Change shape, immunity to sleep and charm effects.
  
• Automatic Languages: Common. Bonus Languages: Auran, Dwarven, Elven, Gnome, Halfling, Giant, Terran.
  
• Favored Class: Rogue.
  
• Level adjustment +4.
  

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System Reference Document -> List of Creatures (SRD)