User blog comment:Crimson and Darkness/Shin DxD 3 discussion/@comment-2601:87:8300:20D0:3886:DF70:BE7:6028-20190823023509/@comment-10330822-20190823033329

I'd assume its designated summoner only. You make a familiar pact with the creature in the night of the full Moon. Usually you go to the special forest for this, however, presumably you can turn pretty much anything into a familiar as long as the pact succeeds. Even Tiamat can apparently be made a familiar under the right conditions.

Usually, the familiars aren't that large or strong, and most devils just use them for small tasks. However, because Ryuuteimaru is a ship, Issei can ride him similar to how Asia can ride her dragon familiars.

Ryuuteimaru was originally a legendary Skíðblaðnir ship given Issei by Surtr Second as congratulatory gift upon becoming a Middle-Class Devil. And it was explained that Ryuuteimaru would evolve as Issei developed.

In a way, its similar to Puu from Yu Yu Hakusho. Ryuuteimaru is Issei's "spirit beast" and develops as Issei does into grander forms. Which is ironic, because presumably as Issei gets more powerful so will Ryuuteimaru, and possibly AxA will become greater overall as a result.