Gyatarô is a convicted criminal plucked from death row to impersonate Shinpachi Nagakura within a substitute Shinsengumi unit formed following the assassination of the original captains. Before his conviction, he led an orphan gang and operated as a notorious crime collector, deeply entrenched in illegal activities. Despite this ruthless past, he displayed a protective and caring nature as a boss figure towards his gang members.

He presents as an older man with darker skin. His thinning, unkempt orange hair is typically tied back, and he often sports a chinstrap beard while missing several teeth. Distinctive features include prominent red tattoos spanning his shoulders and wrists, purple pince-nez glasses perched on his face, and a habitual grip on a golden kiseru pipe. His usual attire consists of a sleeveless brown kosode worn without an underlayer, jika-tabi footwear, and a carried tan purse.

Operating under Shinpachi Nagakura's stolen identity in the substitute unit, Gyatarô wields a unique weapon: a modified flintlock rifle fitted with Shinpachi's broken sword as a bayonet, reflecting his pragmatic approach to combat.

His name, Gyatarô (逆太郎), combines the kanji for "against" or "opposite" (逆) with the common suffix 太郎 (tarou), suggesting meanings like "the boy who went against" or "opposite son," aligning thematically with his defiant and criminal history.