|Mattias "Leo" Bruno|
|Age||Late Teens - Early 20s|
|Melee Weapon||Kannagi Mk 5|
|Ranged Weapon||Arisaka Mk 1|
|Accessory Weapon||EZ Katze I|
Mattias "Leo" Bruno is a healer class sinner, and one of the protagonist's companions.
Wears a pink tag with M.B.
Energenic and full of spirit, Mattias is a rookie sinner who is a good friend to the protagonist but is also his or her rivalry. He often says, "Shaz" and runs his mouth which often leads him into trouble, mostly with Nataila "9" woo. He is also shown to be a ladies' man when he automatically assumes that sinners of the female gender are into him (Even though they are not).
Mattias is a rookie sinner that meets the Protagonist after they are degraded to CODE 1 due to amnesia caused by an Abductor's attack. While initially very competitive with the Protagonist, going as far as to self-proclaim himself their rival, he eventually develops a strong friendship with them.
During their time as CODE 2 sinners, he heard a rumor about a ghost girl inhabiting the LEVEL-2 Cell Garden and urged the protagonist to check out if the rumor was true. After their Accessories forbade them to enter the Cell Garden, a shady sinner named Carlos recommended the protagonist ask his Citizen friend Julien for help. After modifying their Accessories to allow them to enter the garden, he and the protagonist infiltrated the Cell Garden and discovered a girl tied up with black thorns; after the protagonist attempts to free her, they are caught and taken to their cells with an extension to their sentences.
When the protagonist insist on trying again to rescue her, he decides to pass on the offer to avoid another extension of his sentence. After the protagonist rescues the girl, On High launches a retribution on the Panopticon and he, the Protagonist, Uwe and Carlos face off a Ramosa. After defeating it, both he and the Protagonist are automatically promoted to CODE 3.
- Mattias likes using the word "shaz" to express his exclamation, which seems to be created by himself.
- He, alongside Beatrice, Uwe, Natalia and Elfriede are the companion characters that cannot be recreated in the Character Editor.
- Mattias is voiced by Akira Ishida