Familiars

Familiars (使い魔 Tsukaima) are monsters and supernatural beings in High School DxD who have been magically bound to a Devil as a servant or ally.

Abilities
All known Familiars have shown the ability to assume a human form to perform their duties. It's unknown if this is a power common to all Familiars, or an ability that all species of known Familiars possess.

Duties
There do not appear to be any specific duties that a Familiar are supposed to perform. Mostly they have been shown performing janitorial work and odd jobs for their masters. There has also been mention of Asia's Familiar Rassei acting as a bodyguard for her during Rating Games.

Bat(?)
The actual race is currently unknown.
 * ? : Rias Gremory currently has one but it's name is not known at present.

Cat(?)
The actual race is unknown at present.
 *  Shiro  (シロ): Shiro is the familiar of Koneko Toujou. It has white hair and hazel eyes.

Slime
Slime is a gelatinous form with a greenish hue that lives in the Pet Forest. It has no real shape, as its form is almost constantly changing. According to the Familiar Master, Slime is a type of creature that loves to eat clothing (of women in particular).
 *  Sli-Sli : Sli-Sli is the name Issei Hyoudou gave to the slime that ambushed the Occult Research Club before it was destroyed by Rias Gremory and the others.

Sprite Dragon
A Top-tier normal class Dragon that can generate powerful blue lightning. According to the Familiar Master, a Sprite Dragon needs to be tamed as a child or else they will become uncontrollable as adults. A Sprite Dragon is considered as a top-breed Familiar.
 *  Rassei : Rassei is the familiar of Asia Argento. It has blue scales.

Tiny Demon(?)
The actual race is unknown at present.
 *  ? : Akeno Himejima currently has one but it's name is not known at present.

Trivia

 * Dragon King Tiamat is stated to be a potential Familiar.