Ryuuteimaru

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

Appearance
It resembles a toy boat that will eventually grow larger in size along with its master's growth.

History
It is the legendary ship Skíðblaðnir that was 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 Volume 13 short story "The Worries of the Next, Next Heir" (which takes place after Volume 12), Issei is given a familiar, the living Norse flying ship  Skíðblaðnir , by Surtr Second. Although it is only the size of a toy boat,  according to 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

Powers & Abilities
While it initally appears to be a small ship (starting at the size of a toy boat)  that 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. As it grows, Ryuuteimaru will be able to fill the role of transportation  and eventually serve as a living, flying harem palace for Issei.