Doppelganger (CR 3)

Medium Monstrous Humanoid (Shapechanger)
Alignment: Usually neutral
Initiative: +1 (Dex); Senses: Listen +6 and Spot +6

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AC: 15 (+1 Dex, +4 natural), touch 11, flat-footed 14
Hit Dice: 4d8+4 (22 hp)
Fort +4, Ref +5, Will +6

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Speed: 30 ft.
Space: 5 ft./5 ft.
Base Attack +4; Grapple +5
Attack: Slam +5 melee
Full Attack: Slam +5 melee
Damage: Slam 1d6+1
Special Attacks: Detect thoughts

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Abilities: Str 12, Dex 13, Con 12, Int 13, Wis 14, Cha 13
Special Qualities: Change shape, immunity to sleep and charm effects
Feats: Dodge; Great Fortitude
Skills: Bluff +10*, Diplomacy +3, Disguise +9* (+11 acting), Intimidate +3, Listen +6, Sense Motive +6, and Spot +6
Advancement: By character class

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Climate/Terrain: Any land and underground
Organization: Solitary, pair or gang (3-6)
Treasure: Double standard

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Source: Monster Manual

Detect Thoughts (Su): A doppelganger can continuously detect thoughts as the spell cast by an 18th-level sorcerer (save DC 13). It can suppress or resume this ability as a free action.

Alter Self (Su): A doppelganger can assume the shape of any Small or Medium-size humanoid. This works like alter self as cast by an 18th-level sorcerer, but the doppelganger can remain in the chosen form indefinitely. It can assume a new form or return to its own as a standard action.

Immunities (Ex): Doppelgangers are immune to sleep and charm effects.

Skills: A doppelganger receives a +4 racial bonus to Bluff and Disguise checks. When using alter self, a doppelganger receives an additional +10 circumstance bonus to Disguise checks. If it can read an opponent's mind, it gets a further +4 circumstance bonus to Bluff and Disguise checks.

When in its natural form or in the guise of someone unarmed, a doppelganger strikes with its powerful fists. In the shape of a warrior or other armed person, it attacks with whatever weapon is appropriate. In such cases, it uses detect thoughts to employ the same tactics and strategies as the person it is impersonating.