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Keisaku Satō is a fifteen-year-old high school student and is Eita's friend. He likes to skip the class. He eventually helps Margery during her stay in Misaki City.
Keisaku is described as a handsome boy, often referred as "Bishounen". He has brown hair with brown eyes.
He has a very lonely personality, mostly because his parents left him alone long ago in his house, and skips school constantly. When Margery decided to make him his guider, he seem to be more cheerful. He wants to be a Flame Haze to help her in the battle, but he learned and accepted the fact that he doesn't have to be a flame haze to help her. He can be an outlaw member to support her with information. He didn't admit his feeling to her in the first and the second seasons, but it's revealed in the final season, even becoming a part of outlaw.
- He's a close friend of Eita, but after the school festival incident, Eita used to avoids him.
- He's friend with Yuji, Ike, Ogata, and Shana.
- He likes Margery, and he will do anything for her. Event after Margery left him and Eita (but end up returning to them), he still believes on her. He wants to be someone who is useful to her, so that he can follow her, and he become an outlaw member, after studying hard. In the third season, Margery learns the truth behind the silver. Keisaku confesses and kisses her, giving her the new meaning to live forward.
- In the final, seems that he's helped by a flame haze called Rebecca Reed.
- To Margery Daw: "No matter what anyone thinks of you, no matter what you thinks of yourself, the fact that you're here means something to me. My entire existence is devoted to keeping you alive. When I heard that you needed me, I was happy. That happiness told me, that I really do live for your sake."