Reo Itsuki, the ultimate toxicologist is a quiet reserved boy who doesn't seem to care much about the situation. He spends most of his time alone. Though he doesn't intend on making friends he offers his knowledge and support as toxicology isn't his only strong suit, it's a hobby as he states it to be. Calm and quiet at the worst of times people have questioned whether he really does experience emotions and when Reo doesn't want to talk, his puppet, Astrix does the speaking for him. Reo claims he's been studying toxicology since he was 6 but he doesn't talk about himself often due to his quiet nature and another reason which he's hesitant on speaking about. When he finds an inconsistency in someone's wording or theory he won't hesitate to butt in and prove them wrong for which he claims is the sake of helping everyone trust each other.

The voice actor should be quiet and calm, well reserved while sounding monotone, the puppet, Astrix is pretty cheerful and Joyful, being Reo's more optimistic mouthpiece