Reiji (レイジ / アリーア・フォン・レイジ・アスナ)

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A mysterious boy Sei Iori encounters in town. He's overbearing, selfish, quick to anger and lacking in manners. Thrust into the Gunpla Battle cockpit, he shows off incredible piloting skills on his first try.

And has mysterious story about himself being a prince in the another world, Arian Kingdom