Why do we refer to Rengoku by his last name?
For the most part, the Fandom calls most characters by their first name (Mitsuri, Obanai, Tengen, Zenitsu, etc.) Yet Rengoku is one of the few characters who actually has other members of his family who share his last name, yet I rarely ever see anyone call him Kyojuro. He's always just Rengoku, despite there being 2 other Rengokus in the series.