So I saw the bodyguard suggested build in the SRD and thought "That sounds awesome, but you know what will make it even better? If it were a shinobi." Yes, I know very anime that's just how I am. The rune is an obvious problem but giving them a fox mask would build on the anime field and hide the rune as per the rules (see below). Can a bipedal take on a human form or does it have to be a monstrous humanoid or something similar. Maybe an Aasimar or a Humanoid Fey? If not is there a way to accomplish this with feats or archetypes.
The SRD says
The eidolon’s physical appearance is up to the summoner, but it always appears as some sort of fantastical creature. This control is not fine enough to make the eidolon appear like a specific creature. The eidolon also bears a glowing rune that is identical to a rune that appears on the summoner’s forehead as long as the eidolon is summoned. While this rune can be hidden through mundane means, it cannot be concealed through magic that changes appearance, such as alter self or polymorph (although invisibility does conceal it as long as the spell lasts).
Biped Starting Statistics
Size Medium; Speed 30 ft.; AC +2 natural armor; Saves Fort (good), Ref (bad), Will (good); Attack 2 claws (1d4); Ability Scores Str 16, Dex 12, Con 13, Int 7, Wis 10, Cha 11
Claws, limbs (arms), limbs (legs).
Bodyguard Source: PZO1117
The eidolon looks like a humanoid warrior. The natural armor of a Bodyguard eidolon appears to be a suit of metal plate, though this armor is actually part of the eidolon’s body. Bodyguard eidolons are normally trained in a variety of dangerous weapons.
11 points: Base Form biped; Primary Evolutions improved natural armor, weapon training; Secondary Evolutions ability increase (Strength), fast healing, weapon training (martial).
An unchained Eidolon can do that via the Acesstor Subtype and many others which is why I specified chained which from what I've heard is a massive upgrade from unchained.
Ancestor source: PZO9473 Whether created by bonding with the soul of a petitioner that was the summoner’s ancestor in life or calling upon the impressions of such a soul left behind on the Ethereal Plane or the Akashic Record, ancestor eidolons are outsiders cast in the mold of mortals, and exhibit extraordinary skill and talent. They usually appear as idealized versions of the bodies they possessed while still alive, but distinctive and proud. Ancestor eidolons lend whatever aid they can to their summoner, who typically has some form of familial connection with the eidolon, be it by blood, honor, or devotion.