Joker shows up during the Circus Arc as a key figure in Noah's Ark Circus, the traveling show caught up in the missing children case. At first he seems to run the whole operation, but it turns out he's more of a hired manager—basically the person keeping things running day-to-day. Still, most of the circus crew treats him like he's in charge since he's the one actually directing the main performances.
One thing that stands out about Joker is his missing hand, lost sometime in his youth under unclear circumstances. He replaced it with a prosthetic hand that the circus doctor built for him, which he uses to keep doing his job without missing a beat.
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