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Hero tickets are a kind of currency that allows the player to use the 'Hero Gacha' in order to obtain new heroes and costumes.
It can be obtained from:
Drawing Card Gachas (usually 50 draws = 1 Hero Ticket. Sometimes Hero Obtain Assist Gacha will be open, which is 25 draws = 1 Ticket. also, SR+ Guaranteed UR 15% Gacha, the paid BM only event gacha, is 11 draws = 1 Ticket).