Ryuuteimaru initially bears a resemblance to a toy boat, but will eventually change in size and shape along with Issei's growth.
By Volume 17, Ryuuteimaru was large enough to allow Asia and Rossweisse to ride on it around the battlefield.
As of Volume 22, Ryuuteimaru has grown into an enormous flying ship adorned with the design of a dragon, capable of carrying more than several people.
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.
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.
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.
In Volume 17, Ryuuteimaru was summoned to support D×D against the battle with Qlippoth. Its size has changed from the size of a model toy to a ship. Ryuuteimaru was used to transport Asia, with Rossweisse guarding her, from one place to another through the battlefield to help the wounded.
Red Dragon Emperor of the Blazing Truth × White Dragon Emperor of the Morning Star: The True Dragon(s) of the Kuoh AcademyEdit
In Volume 22, Ryuuteimaru was summoned by Issei to transport him and his peerage members to the opening ceremony of the Azazel Cup. Due to the change in Issei from Diabolos Dragon.G, its size has changed once again to an enormous ship.
Powers & AbilitiesEdit
Growth: Ryuuteimaru will grow and evolve into different forms by using the aura and imagination of its master as its source.
Immense Strength: 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 him during the battle with the Stray Magicians.
Flight: Ryuuteimaru is able to hover and levitate itself.