Fuego looks like a small ball of fire. It's possibly the smallest familiar we see, fitting in Midou's palm perfectly.
Fuego initially appeared in small dosis: the familiar is the one who transfers Tenma's last words to Midou and who burns him to death in the end. Then, Midou also uses it to make an empty building explode and to trap Chihaya Ohtori and Kotarou Tennouji inside the Kazamatsuri Academy gym.
Fuego participates in the battle of Midou versus Kotarou and Chihaya, and it's the one responsible for the doors being blocked after all the other students left the building. It becomes a substance that shields Midou and that Kotarou can't bypass without getting burnt, but eventually, Kotarou finds a way around it. When its summoner decides that he won't accept Kotarou's proposition to be allowed to live on, Fuego burns him to death.
Later on in the route, when Sakuya Ohtori has morphed into his last familiar form, Fuego appears out of nowhere, apparently prompted by late Midou's memory, and aids Kotarou.
- Fire control: Fuego can light things on fire or make them explode in a heartbeat.
- Hardening into a shield-like substance: used to keep Kotarou from getting close to Midou and to keep the other students out of the gym while the battle takes place.