I was watching "Jekkyl" last night, and recently saw "The Avengers" featuring the Hulk, and got inspired.
HYDDENFOLK
Hyddenfolk normally appear to be humans, but tend to look more predatory, particularly when they are manifesting their mutagenic psyche.
+2 to One Ability Score: Hyddenfolk characters get a +2 bonus to one ability score of their choice at creation to represent their varied nature.
Size: Hyddenfolk are Medium creatures and have no bonuses or penalties due to their size. If a hyddenfolk’s size changes to Small due to her mutagenic psyche, she gains a +1 size bonus to her AC, a +1 size bonus on attack rolls, a –1 penalty to her Combat Maneuver Bonus (CMB) and Combat Maneuver Defense (CMD), and a +4 size bonus on Stealth checks.
Speed: Hyddenfolk have a base speed of 30 feet.
Hidden Nature: Hyddenfolk receive a +2 racial bonus to Bluff, Disguise, and Stealth skill checks due to their deceptive natures.
Human Blood: Hyddenfolk count as humans for any effect related to race.
Mutagenic Psyche (Ex): A hyddenfolk’s mutagenic psyche is an alter ego that has a different personality than her normal form; it takes the form of a Small or Medium-sized humanoid creature. The mutagenic psyche often shares memories and basic goals with the hyddenfolk’s normal personality but goes about meeting those goals in a different manner. The mutagenic psyche is often ugly and monstrous, but just as often the changes are subtle and barely noticeable, or they may look as different as two unrelated people. Indeed, the mutagenic psyche often has his or her own name, and may attempt to maintain independent relationships and strongholds (though the alter ego’s limited time in existence often makes this difficult). The mutagenic psyche even has his or her own alignment (which is selected by the player, but must be different on both the ethical axis and moral axis from the hyddenfolk‘s normal alignment). The change in alignment only affects the hyddenfolk while she is manifesting her mutagenic psyche.
The hyddenfolk’s mutagenic psyche also has her own ability scores, character classes, class features, favored class bonuses, feats, languages, and skills; she is essentially a totally different character. Her ability scores are generated the same way the normal version of the hyddenfolk generated her ability scores (point buy, randomly rolled, elite array, etc.), she may choose to place her level-based ability score increases in the same or different ability scores as the normal version, and the mutagenic psyche can place her +2 racial bonus to her ability score in the same or a different ability than the normal version of the hyddenfolk. This ability otherwise functions as alter self. The hyddenfolk’s mutagenic psyche may select the following alchemist discoveries as feats, using his character level as his effective alchemist level.
Cognatogen, Feral Mutagen, Grand Cognatogen, Grand Mutagen, Greater Cognatogen, Greater Mutagen, Healing Touch, Lingering Spirit, Mummification, Mutagen, Nauseating Flesh, Parasitic Twin, Preserve Organs, Spontaneous Healing, Tentacle, Tumor Familiar, Vestigial Arm, Wings.
Once per day, usually at sundown or midnight, the hyddenfolk must make a DC 20 Will save or her mutagenic psyche manifests. For each night the hyddenfolk successfully makes her Will save, the DC increases by a cumulative +1 the following night. The hyddenfolk’s mutagenic psyche persists for 2d6 hours.
In order to access her mutagenic psyche voluntarily, the hyddenfolk must use a standard action and make a DC 20 Will save.
A hyddenfolk may be forced to assume her mutagenic psyche against her will by stress or damage. Anytime the character is in her normal form, she may be forced to switch forms after suffering a critical hit or failing a Fortitude save. In these situations the hyddenfolk must make a DC 20 Will save; if she fails, on her next turn she uses a standard action to change to her mutagenic psyche.
Languages: Hyddenfolk begin play speaking Common. Hyddenfolk with high Intelligence scores can choose any languages they want (except secret languages, such as Druidic).
The hyddenfolk can select from the following racial feats.
Amphibious Psyche
Prerequisites: Swim 1 rank, hyddenfolk
Benefit: You gain a Swim Speed of 30 feet, a +8 racial bonus on Swim checks, and can take 10 on Swim skill checks when you assume your mutagenic psyche.
Arcane Psyche
Prerequisites: any metamagic feat, Cha 13+, Int 13+, Spellcraft 5 ranks, hyddenfolk
Benefit: Your mutagenic psyche can apply any one metamagic feat that you know to a spell you are about to cast. This does not alter the level of the spell or the casting time. You can use this ability once per day for every 2 ranks in Spellcraft that you possess. Any time you use this ability to apply a metamagic feat that increases the spell level by more than 1, you must use an additional daily usage for each level above 1 that the feat adds to the spell. Even though this ability does not modify the spell's actual level, you cannot use this ability to cast a spell whose modified spell level would be above the level of the highest-level spell that you are capable of casting.
Brutal Psyche
Prerequisites: BAB: +5, Str 13+, proficient in all simple weapons, hyddenfolk
Benefit: You add +2 to damage rolls with all simple weapons.
Creeping Psyche
Prerequisites: Climb 3 ranks, hyddenfolk
Benefit: You gain the benefits of spider climb whenever you assume your mutagenic psyche. This is an extraordinary ability.
Divine Psyche
Prerequisites: channel energy 3d6, hyddenfolk
Benefit: You add +1 per die of you channel energy ability whenever you assume your mutagenic psyche.
Durable Psyche
Prerequisites: Thick-skinned Psyche, Con 15+, hyddenfolk
Benefit: You gain a DR 1/- when you assume your mutagenic psyche.
Special: You may select this feat multiple times; its effects stack.
Enlarged Psyche
Prerequisites: BAB: +1, hyddenfolk
Benefit: You gain the benefits of enlarge person whenever you assume your mutagenic psyche. This is an extraordinary ability.
Hasty Psyche
Prerequisites: Acrobatics 5 ranks, Dex 15+, hyddenfolk
Benefit: You can use haste a number of rounds per day equal to half your ranks in Acrobatics; activating this ability is a swift action and the rounds do not need to be consecutive.
Huge Pysche
Prerequisites: BAB +14, Enlarged Psyche, hyddenfolk
Benefit: You gain the benefits of giant form 1 whenever you assume your mutagenic psyche. This improves to giant form 2 when your BAB improves to +16. This is an extraordinary ability.
Improved Amphibious Psyche
Prerequisites: Amphibious Psyche, Swim 3 ranks, hyddenfolk
Benefit: You gain a bonus on Constitution checks to hold your breath equal to the number of ranks in Swim you have. When you have 5 ranks in Swim, you gain the ability to breathe water.
Improved Visionary Psyche
Prerequisites: Visionary Psyche,, Perception 5 ranks, Wis 15+, hyddenfolk
Benefit: You can use see invisiblity a number of rounds per day equal to half your ranks in Perception; activating this ability is a swift action and the rounds do not need to be consecutive.
Mighty Psyche
Prerequisites: BAB: +1, Str 13+, hyddenfolk
Benefit: You gain a +1 bonus to your CMB during your mutagenic psyche.
Multiple Personalities
Prerequisites: Character level 3, Cha 17+, hyddenfolk
Benefit: You gain an additional number of mutagenic psyches equal to your Charisma modifier minus 2.
Pure Psyche
Prerequisites: Cha 13+, hyddenfolk
Benefit: You gain a +1 bonus on all saving throws during your mutagenic psyche.
Quick Psyche
Prerequisites: Dex 13+, hyddenfolk
Benefit: Your speed for all your movement abilities increases by +10 feet when you assume your mutagenic psyche.
Resistant Psyche
Prerequisites: Con 13+, hyddenfolk
Benefit: Select acid, cold, electricity, fire, or sonic. You gain energy resistance 5 against the selected energy type.
Special: You may select this feat multiple times. Each time you do, you may select a different energy type or the same. If you select an energy type multiple times, your resistance to that energy type stacks.
Steady Psyche
Prerequisites: Con 13+, hyddenfolk
Benefit: You add your Constitution modifier to your Combat Maneuver Defense.
Stealthy Psyche
Prerequisites: Stealth 7 ranks, Dex 15+, hyddenfolk
Benefit: You can use invisiblity a number of minutes per day equal to half your ranks in Acrobatics; activating this ability is a swift action and the minutes do not need to be consecutive.
Swift Psyche
Prerequisites: Con 13+, hyddenfolk
Benefit: You can assume your mutagenic psyche as a swift action.
Normal: You can assume your mutagenic psyche as a standard action
Thick-skinned Psyche
Prerequisites: Con 13+, hyddenfolk
Benefit: You gain a +1 natural armor class when you assume your mutagenic psyche.
Special: You may select this feat multiple times; its effects stack.
Visionary Psyche
Prerequisites: Perception 3 ranks, Wis 13+, hyddenfolk
Benefit: You gain darkvision with a range of 60 feet when you manifest your mutagenic psyche.
Wild Psyche
Prerequisites: wildshape, hyddenfolk
Benefit: Whenever you wildshape while manifesting your mutagenic psyche, you gain a +2 bonus to your choice of Strength, Dexterity, or Constitution..