| Koja of Khazari |
Here's my conversion of the greater doppelganger. I went with a doppelganger variant instead of a full monster build because, for one thing, it is a variant doppelganger, and treating it as such seems to be in line with the Pathfinder design philosophy. As presented, its base CR will be a good deal lower than the 3rd edition version. I don't see this as a problem in itself, considering that its CR will be adjusted upward if it absorbs a particularly powerful personality. In fact, one of the problems with the original monster was that its power could vary widely depending on the identities it had consumed, yet it had a flat CR of 12.
I also attempted to address some other areas that were either vague, nonsensical, or clumsy. I think the result works pretty well.
Doppelganger variant- Mirrorkin (Greater Doppelganger)
A greater doppelganger, also known as a mirrorkin, is a normal doppelganger with the advanced simple template, as well as a number of additional abilities. A mirrorkin will frequently have levels in a character class as well.
Challenge rating: As a normal doppelganger +2 (includes advanced simple template). When calculating the CR of an individual mirrorkin, the class levels of its already consumed identities (see consume identity ability) should be factored in to the overall CR of the encounter. Add +1 to the mirrorkin's CR for every 2 levels possessed by the highest level identity the mirrorkin has consumed (excluding divine spellcasting classes).
Defensive abilities: Instead of immunity to charm and sleep, a mirrorkin gains immunity to all mind-affecting effects. In addition, a mirrorkin is immune to any effect that detects alignment. If the mirrorkin is wearing the guise of an identity it has consumed (see consume identity ability), it may choose to reveal the identity's alignment or keep it hidden.
Special attacks: A mirrorkin gains the following special attacks.
Polymorph Rend (Su): A mirrorkin can make a touch attack against a creature that is under the effect of a polymorph spell or effect, or any other shapechanging ability. If successful, this touch attack deals 6d6 points of damage, and causes the creature to revert to its natural form unless it succeeds on a Fortitude save. The save DC for this ability is 10 + 1/2 the mirrorkin's HD + the mirrorkin's Cha modifier.
Consume Identity (Su): A mirrorkin can absorb the mind, memories, and personality of other humanoid creatures of Small or Medium size. To do so, it must spend at least 1 minute in physical contact with its victim. In general, this means the victim must be helpless. This process doesn't remove the victim's memories and personality, though it does severely damage the victim's mind, dealing 2d6 points of damage to the victim's Intelligence, Wisdom, and Charisma (roll separately for each). If the victim is conscious for this process, it is an extremely traumatic experience, as his thoughts, memories, emotions, and even his basic sense of self are violated against his will. If the intended victim is dead, the mirrorkin can still consume his identity. To do so, the mirrorkin must consume the victim's brain within 1 hour of his death.
Once the mirrorkin has consumed a victim's identity, it can then assume the victim's form with total accuracy. The mirrorkin possesses all the victim's memories, personality, and even abilities. Essentially, the mirrorkin now is that person; racial abilities, class levels, hit dice, ability scores, skills, feats, even alignment are matched perfectly, and even the victim's closest friends and family can't tell the difference (no disguise check is needed; the copy is that perfect). The only exception to this is divine spells and divinely granted abilities, such as a paladin's lay on hands ability. For this reason, mirrorkin rarely consume the identities of such characters. (As a rule of thumb, any class ability that can be lost by violating a code of ethics cannot be emulated by a mirrorkin assuming the identity of one of that class.) The mirrorkin still has access to its own abilities, including any class abilities it may have. At any time, it may use its own ability scores rather than those of its assumed identity (this is a deliberate act; it will never do so accidentally).
The mirrorkin always uses either its own hp total (modified by its current Con score) or the hp total of its assumed identity, whichever is higher. Any damage the mirrorkin has taken in one form remains if it changes form before being killed. If the damage taken by the previous form exceeds the maximum hp of the new form, the mirrorkin does not die immediately; the newly assumed form (whether consumed identity, normal shapechange, or natural form) will always have at least 1 hp per hit die.
[I'm not sure I like handling hp this way. My original idea was to always use the mirrorkin's own hp total, modified by its current Con, but that sort of falls apart when assuming the identity of, say, a 14th-level fighter, while still having an average of 34 hp or so. The way I have it now should work pretty well, and the few weird corner cases it produces are neither likely nor particularly problematic, but I admit it feels a bit clunky, and I'm open to suggestions for improving it.]
A mirrorkin can hold a number of identities equal to 3 + its Cha modifier. If it ever exceeds this limit, it must immediately lose one identity of its choice. Mirrorkin with the resources to do so might keep their victims alive in captivity, so they can later absorb their identity again.