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partner (esp. in manzai); companion
partner for the night (e.g. at a brothel)
相方との息が合ってきた。
I'm getting in sync with my partner.
彼女は相方と十年以上組んでいる。
She has been partnered with her co-performer for over ten years.
舞台で相方のボケに素早くツッコんだ。
I quickly responded to my partner's gag on stage.