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.

Devils may summon their Familiars to them at any time.

Duties
There do not appear to be any specific duties that a Familiar are supposed to perform, though Rias has stated they can be used to bring information or carry messages. 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.

Potential Familiars
Other races have been mentioned as being possible candiates for Familiars. These include: {| border="1" cellpadding="1" cellspacing="1" class="article-table" style="width: 650px;" ! scope="col" style="width: 650px; background-color: rgb(120, 0, 0); border-color: black; height: 10px;"|

Dragon Kings
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 * Tiamat:
 * Tiamat:

Slime

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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) and women's secretions. ! scope="col" style="width: 650px; background-color: rgb(120, 0, 0); border-color: black; height: 10px;"|
 * Slime-taro: Slime-taro 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. Since it was destroyed before Issei could make a contract with it, he could not make it into his familiar.

Undines
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 * style="text-align: left; vertical-align: top; width: 650xp; background-color: rgb(50, 0, 0); border-color: black;"|Undines are described as humanoid with blue sparkling hair, and having the body of a male martial artist. Undines are said to be a water-type Seirei (Holy Spirit).
 * style="text-align: left; vertical-align: top; width: 650xp; background-color: rgb(50, 0, 0); border-color: black;"|Undines are described as humanoid with blue sparkling hair, and having the body of a male martial artist. Undines are said to be a water-type Seirei (Holy Spirit).

Nine-Headed Hydra

 * style="text-align: left; vertical-align: top; width: 650xp; background-color: rgb(50, 0, 0); border-color: black;"|The Nine-Headed Hydra's greatest features are its deadly poison that even Devils fear, and its immortal body. The Familiar Master touts it as one of the most evil pets who will even kill its master.
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How to Obtain
On the night of a full Moon, Devils will go to a special forest and, with the guidance of a Familiar Master, capture and bind a Familiar they find suitable.

Trivia

 * In the anime, Familiars are called Pets.
 * Gaining a Familiar is stated to be part of being a good Devil.
 * In the Light Novels, it is suggested one can have multiple Familiars.