Well, yeah, "rude" is a way to put it. More simply that Kenpachi has an utter disregard for social conventions like honorifics.
And I just looked up a scene and it confirmed what I thought, Kenpachi calls Ikkaku just "Ikkaku". Of course, he met Ikkaku and Yumichika back in Rukongai, so he's on pretty familiar terms with them. They usually refer to him just as "captain", though, showing some respect, but Kenpachi's calling them by name like that is more a familiar thing than a rude thing.
...hm, he also calls Orihime "onna" when asking her for directions when running around the Seireitei.
Anyway, the men in Kenpachi's division would just call him "taichou" or "Zaraki-taichou". Yachiru's the only one who'd call him any different, and she calls him Ken-chan. Unless you mean what they'd call everyone else... that'd depend on the individual. Makizou (or Maki-maki) tends to use more honorifics because he's a lower-ranking 11th divisioner and has no intention of pissing off the higher officers who could probably kill him without breaking a sweat. So generally speaking, probably most the men in 11th use honorifics based on a combination of how much the respect someone, and/or how much they fear/are intimidated by them.
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And I just looked up a scene and it confirmed what I thought, Kenpachi calls Ikkaku just "Ikkaku". Of course, he met Ikkaku and Yumichika back in Rukongai, so he's on pretty familiar terms with them. They usually refer to him just as "captain", though, showing some respect, but Kenpachi's calling them by name like that is more a familiar thing than a rude thing.
...hm, he also calls Orihime "onna" when asking her for directions when running around the Seireitei.
Anyway, the men in Kenpachi's division would just call him "taichou" or "Zaraki-taichou". Yachiru's the only one who'd call him any different, and she calls him Ken-chan. Unless you mean what they'd call everyone else... that'd depend on the individual. Makizou (or Maki-maki) tends to use more honorifics because he's a lower-ranking 11th divisioner and has no intention of pissing off the higher officers who could probably kill him without breaking a sweat. So generally speaking, probably most the men in 11th use honorifics based on a combination of how much the respect someone, and/or how much they fear/are intimidated by them.