Ryuuteimaru

Ryuuteimaru is the legendary ship Skíðblaðnir and the familiar of Issei Hyoudou.

Appearance
Ryuuteimaru initially bears a resemblance to a toy boat, but will eventually grow larger in size along with Issei's growth.

History
Ryuuteimaru is one of the legendary Skíðblaðnir ships that were created by the tribe called Ívaldi who also created Thor's Mjölnir. Supposedly, very few of these ships were created.

The Heroic Oppai Dragon
In the Extra Life of Volume 13, "The Worries of the Next-Next Heir" (which takes place after Volume 12), Issei was given a familiar, the living Norse flying ship Skíðblaðnir, by Surtr Second. Although it is only the size of a toy boat, according to MacGregor Mathers, it will continue to grow and evolve into different forms by using the aura and imagination of its master as its source. Second also jokes that if Issei is going to become a "Harem King", he would need a harem palace. With Issei as its master, it is highly possible for it to actually grow into a flying harem palace.

The Legend of Oppai Dragon and his Lively Companions
It was briefly mentioned by Issei when he was discussing about familiars with Asia. He states that he made a master and servant relationship with his familiar, Skíðblaðnir, which shows some compatibility between the two. Issei later officially names it Ryuuteimaru in Volume 14.

Ryuuteimaru later makes an appearance during the confrontation with the Stray Magicians from Khaos Brigade when Issei summons it as means of transportation as he is still unable to support himself with his Devil wings.

Powers & Abilities
While it initally appears to be a small ship (starting at the size of a toy boat), it will continue to grow and evolve into different forms by using the aura and imagination of its master as its source.

Despite its small size, it can still fly while carrying its master. Issei states that it has amazing horsepower and strength which was demonstrated when it was able to carry its master during the battle with the Stray Magicians.

Trivia

 * When Ryuuteimaru was given to Issei, it was stated that it could potentially evolve into its own dimensional plane.