Agate is a purely physical character with high strength. In the first installment, he's relatively less useful due to his poor combination of magic skills and rather limited selection of combat skills. His crafts are still useful as he contains an ability to increase his CP gauge at the cost of some of his (already very high) HP. This allows him to perform many crafts very quickly and gives him his pure physical and warrior status.
However, from the second installment, he becomes incredibly powerful as one of his skill's new level allows him to basically spam S-Crafts indefinitely, along with a new S-Craft called "Dragon Dive" which deals on all enemies. He thus rapidly becomes one of the best combat-oriented characters in the game.
Agate's Orbment consists of three lines which greatly limits his arts' potential, while oppositely increasing parameters. Two of his slots require Fire Quartz. Even with using fire arts, he would never be able to get into the high-level arts due to his orbment structure.