Dark Yashiro is a Mirage that, through the effect of being within close proximity to a Dragonstone shard, was affected by a residual essence of Yashiro Tsurugi in his Carnage Form and transformed into his warped doppelganger. Believing himself to be the true Yashiro, he holds a concert in the Idolasphere, and Itsuki's party puts a stop to him in order to recover the piece of the Dragonstone in his possession.
Though they are physically near identical, Dark Yashiro's personality deviates from the real Yashiro's in a number of ways. One which Yashiro finds appalling in particular is Dark Yashiro's tendency to use words and phrases like "Ta-ta" in his speech, which the real Yashiro would never use.
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Dark Yashiro features a unique trait in his boss battle, in that he can separate himself into multiple copies for several turns at a time before reverting to a single body. Early in the fight, he splits into two, but later, he is able to split into as many as four. If a duplicate is defeated, however, it will not regenerate the next time Yashiro copies himself.