Also known in Sanskrit as 'bīja' or 'seed', these 'seed syllables' are thought to be connected to spiritual principles in Esoteric Buddhism and are used in mantras. Bai is one example.
Glossary: shuji
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shuji (種字)
Referenced from: Mirage 4, Mirage: Saiai no Anata e, Mirage 6, Mirage 8, Mirage 9, Mirage 12, Mirage 16, Mirage: Shinku no Hata wo Hirugaese, Mirage: Exaudi Nos