ナイン

Episode 3 and 4 spoilers.
An alternate version of Kakeru from a distant world.

Implied to be the player and is the true owner of Overload. Is able to aid Kakeru through Overload due to being an alternate version of him, and is regarded by Sophitia as the ninth and final Artifact User required to defeat Iris.

He is first introduced/revealed to have existed in Haruiro and plays a major role in defeating Iris in Yukiiro.

Kakeru becomes aware of Nine near the end of Yukiiro, and calls him 相棒/Partner.