Mana-Book 1 of The Mana Saga by Asher Tensei
Eurich can pass quizzes, but can he pass through a barrier to save his world? Seventeen year old Eurich Aurion can add one more thing to his high school to-do list; become Dragoon Oblivion, a winged warrior with the soul of a dragon. A decaying seal placed on Omni Chaos, a peerless warlord, is causing Earth to merge with a six-mooned realm called Samsera. As the seal between worlds weakens, Eurich finds he's not alone in his connection with Samsera. Raine, his closest friend, uncovers her own powers as a sorcerer. With her aid, he must cross the barrier, take on Chaos, and stop the merge. Not only can she see the seal dividing the worlds, she may have the ability to traverse through it. Though, if Eurich allows Raine to embrace such power, she may become a valkyrie, an ancient predator of the dragoons. Eurich must either choose to cross the barrier, and save two worlds from Chaos’ wrath, or risk it all for the sake of a girl he loves—because just as Earth and Samsera can never be together, neither can dragoon and valkyrie… Life in Roanoke was supposed to be simple.