Arimori was born into the Tsuchimikado house as the son of the thirty ninth head Exorcist Arima Tsuchimikado and his wife Mana. As a young child, he was very close to his parents and sought to be a powerful Exorcist like his father.
At some point, he skipped elementary school and enrolled in the Seiyouin Exorcist Academy.
(Source: Sousei no Onmyouji Wikia)
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