Tetsu Amamiya

A high school student who secretly has a shapeshifting creature named Cenco.

Artificial Limbs: Cenco eats Tetsu's arm to recreate his own after it gets blown off. After the battle, Cenco shapeshifts into an arm to replace Tetsu's one.
Ordinary High-School Student
The Slacker: Somewhat gives off this vibe, personality-wise.

Cenco

A shapeshifting creatured controlled by Tetsu.

Because You Were Nice to Me: He didn't hesitate at all to relinquish Tetsu's rights to control him over to Yuki because Yuki feeds him and treats him a bit better than his original controller, Tetsu.
Extreme Omnivore: Shown to eat chocolate, pudding, metal scraps, and Tetsu's arm.
Mon
Shapeshifting: Cenco is mentioned to be one of the more unique mons because of his ability to transform.
Teleporters and Transporters
Undying Loyalty: Not to Tetsu, but to Yuki. Cenco quickly rushed in to rescue her when he learned that Shu kidnapped her without Tetsu consenting to the action.

A girl at Tetsu's school who discovers Cenco during an encounter with Tetsu.

Action Girl: Towards the end of the film.
Combat Pragmatist: She's the only one who really thinks outside of the box when it comes to the creatures' potential transformations. She even has Cenco turn into a baseball bat at the end of the film and knocks out Shu with it.
Damsel out of Distress: Gets cornered by Shu to pass a message to Tetsu, and then gets captured by Shu as a hostage to lure Tetsu out. But this backfires on Shu twice: though he manages to lure Tetsu and Cenco out and his much larger shapeshifting creature manages to consume Cenco to absorb his powers the first time, he failed to take into account Yuki still being inside Cenco when Cenco was absorbed, leading to his Mon being (accidentally) controlled by Yuki instead. The second time he corners her to force her to give him back his control over his bigger Mon she knocks him out with a Cenco-turned-bat and escapes with Tetsu and Cenco in the process.
Deadpan Snarker: Is all too nonchalant about being kidnapped by Shu, even insulting him to his face without showing signs of being scared of him or his Mon. She also regularly snarks at Tetsu.
Ordinary High-School Student
Plucky Girl
Sweet Tooth: Her penchant for carrying sweets around her all the time implies that she's this.

A young man with two shapeshifting creatures like Cenco. Shu wants Tetsu's powers for himself.