Born in Ravennue Village, Agate Crosner lived with his little sister and his parents. On his 14th birthday, his sister decided to give him a necklace as a gift. Tragically, the village was attacked during the Hundred Days War where his sister was found dead. Since then after the war, Agate gained an indefinete hatred against the Royal Army due to General Morgan's refusal to defend the village.
Chapter 1: Disappearance of the Linde
Agate makes a short cameo when he meets Scherazard Harvey, Estelle Bright, and Joshua Bright leaving Ravennue Village on grunt work. Noticing the Bright siblings as novice bracers, he notes Joshua's capabilities despite his inexperience, but reprimands Estelle's abilities and juvenile mentality.
Chapter 2: Madrigal of the White Magnolia
Agate helps Estelle and Joshua fend off a monster invasion at Krone Pass as the siblings are moving on to Ruan. He remains absent in the chapter until Estelle, Joshua, and Kloe Rinz confront the Ravens to save a local orphan named Clem. Regardless of them being his former crew, Agate follow his priorities as a Bracer by defeating and apprehends them. Under further investigation of the Mercia Orphanage arson, the case was reassigned to him despite Estelle's protests.
Agate is later seen chasing mysterious soldiers outside the inn and everybody joins his search. With Seig's help, they spot the soldiers at the old lighthouse. The group rencounters the Raven gang who were somehow powered-u, and continue forward till they reached the top. They discover that Gilbert Stein, the mayor's aide, had some connection with the arson, only to be double-crossed and stabbed by the mysterious soldiers. The soldiers escape with Agate pursuing them in anger, while leaving Estelle and Joshua orders to report the situation back at the Ruan Bracer Guild.
Chapter 3: The Black Orbment
Agate continues pursuing the mysterious soldiers, but loses the trail until he reached Zeiss. He reencounters the party in the smoking Zeiss Central Factory, where he meets Tita Russell for the first time. After learning that Tita's grandfather, Professor Russell was kidnapped by the same black-clad soldiers, disguising themselves as Royal Army soldiers to evade the public, Agate decides to team up with Estelle and Joshua again. They learn that the black-clad soldiers are at Carnelia Tower after chasing Professor Alba out.
When they reached Carnelia Tower, the group fights the same dobermans at Krone Pass till they reached the top. They finally cornered the black-clad soldiers after a hard fight, forcing them to hasten their airship escape route. Right before the party runs at the best possible time to rescue the professor, Tita appears and shoots the airship. In retaliation, the soldiers shoot a poison bullet at Tita where Agate takes the bullet while the soldiers make their escape. Agate berates and slaps Tita for being an insolent child who can't logically let the bracers do the work where they could've been successful. After the party scales down the tower and reaches the Ritter Roadway, they meet Zane, and Agate soon collapses from the poison.
The party brings Agate to the ZCF infirmary and locate some Zemuria Moss for the antidote. After being treated, Tita remains behind to watch after him while spurting out his "sister's name" in his sleep. When Agate recovers, he was briefed about Professor Russell's current location somewhere within Leiston Fortress. The party decides to stealthy infiltrate the fortress by using an orbal device to avoid detection while hiding in the cargo. On the airship ride, Tita asks Agate about who Micsha is where Agate replies as his little sister from his home village.
After a successful arrival, they soon find Professor Russell in the R&D section talking to Cid, Amalthea, and Richard. The three want Russell to experiment on the Black Orbment for more info on it to which Russell refuses. The party easily dispatches the black-clad soldiers guarding the door and retrieve Russell, only to set off the alarm and meet the Sky Bandits by accident on one occasion. However, Cid (whose family is held hostage) helps them escape. the whole group decides to split up with Agate, Tita, and Professor Russell in hiding while Estelle and Joshua head to Grancel and alert the Bracer Guild of the situation.
Final Chapter: Turmoil in the Royal City
While traveling away from the Intelligence Division with Professor Russell and Tita, Agate sneaks them all on an airliner to Grancel rendezvousing with the rest of the party members right before the Aureole Ruins. Agate's group catches up with the events that happened earlier from retaking the castle and that the conspirators Colonel Richard and Amalthea escaped into the bottom realms.
Agate can be one of the playable characters to assist Estelle and Joshua in the final dungeon if chosen. The group finally arrives to Richard activating up the Aureole with the Black Orbment, but manages to defeat him and the guard robot, Reverie with Cassius's timely arrival.
During the festival, Agate is seen drinking with Zane and contemplating that he should join next years tournament to further his skills against stronger opponents like Zane. After the dinner at the castle, Agate is talking with Tita where he reveals to the player that he was from Ravennue Village.
In Chapter 5, Agate's tough personality is explained. After defeated by Leonhardt, he loses his senses but saved thanks to Tita and was taken back to Ravennue for treatment. Agate opens up his tragic past and the loss of his sister. This begins to establish a very good relationship between him and Tita. After liberating Ragnard of the Black Orbment's control, Agate returns to Rvennue to the village's memorial stone of those deceased. General Morgan appears admitting he blamed himself for what had happened in the past. Leonhardt even appears with flowers where Agate stands but lays down as Leonhardt asks about disturbing those who lay. Leonhardt tells Agate he no longer feels any weakness within him.
The sixth Star Door shows us an event taking place after the events of SC. Agate brings the Ravens gang to Regart's old lair to commence their bracer training. The gang has to reach the top of the mountain where Agate will be waiting for the real test. A dispute occurs in the group but they managed to settle the problem and eventually making it through the top and slightly injuring Agate. This test came as the culminating point of their bracer training.