Agate Crosner, known as the "Heavy Blade," serves as a Bracer across Liberl and Erebonia. Born in Ravennue village near Bose, his peaceful life shattered during the Hundred Days War when his hometown was devastated. This conflict claimed his younger sister, an event permanently marking his character. Disillusioned by the Royal Army's failure to protect the village, he later founded the delinquent Ravens gang in Ruan. His path shifted dramatically after encountering Cassius Bright, the renowned S-Rank Bracer, who inspired him to seek true strength for the greater good. Agate consequently joined the Bracer Guild, rising to A-Rank after his pivotal role in resolving the Calamity of Liberl.

Agate initially presents as blunt, ill-mannered, and harsh, particularly towards inexperienced Bracers like Estelle and Joshua Bright, treating them with severity. Beneath this abrasive exterior lies a kind, caring, and selfless core, embodying the "jerk with a heart of gold" archetype. His war trauma forged fierce independence and reluctance to collaborate, especially with children who evoke memories of his sister. He channels rage into his swordsmanship, though figures like Loewe and Ragnard noted this anger initially hindered his growth. As a Bracer, he demonstrates undeniable skill and reliability, earning deep trust from colleagues. He approaches missions with a calm, analytical mind despite hot-headed tendencies, though his brashness sometimes leads him into danger.

Significant development occurs through his relationships with Estelle, Joshua, and particularly Tita Russell. The young engineer Tita reminds him of his deceased sister, prompting protective, brotherly concern. Their bond strengthens during shared journeys; he supports her engineering experiments despite objections from her suspicious mother, Erika Russell, who accuses him of "corrupting" her daughter. This dynamic fuels frequent clashes with Erika, unaware Agate sees Tita as a sister. These connections help Agate gradually confront his past trauma, accept his sister's death, and reveal a softer, supportive side. His evolution is mirrored physically: early depictions feature spiky hair, long sideburns, and a constant bandana, while later appearances show shorter hair without the bandana, a black long-sleeved shirt, and a green vest, retaining the stone pendant on his chest.

Four years after the Calamity, Agate enters Erebonia following its civil war. Prompted by Prince Olivert's intel on Ouroboros and jaeger movements, he aids understaffed Imperial Bracer branches. His motivations include supporting Olivert, monitoring Tita's enrollment at Thors Branch Campus, and independently investigating the Hundred Days War's truth. He later assists Class VII at Thors and stands with allies like Estelle against the Great Twilight. His combat prowess centers on wielding a massive sword with ease, living up to the "Heavy Blade" moniker. Agate remains one of the series' most frequently playable characters, appearing in six installments.