I did search for this fight and found it but it had no rules and stuff and since bleach had a time skip why not do it over
no prep , in character , both are current , both have their swords
Could honestly go either way in my personal opinion. Kenpachi has the advantage in close quarters combat where any blow he delivers to Byakuya while in Kendo stance would be enough to seriously cripple or even take out Byakuya. However, Kenpachi's means of ranged combat are rather limited compared to Byakuya's as the latter has Hados and the flower petal attacks from his zankpatou to milk which he could use to wear out Kenpachi's body considering the dangerous overconfidence the big man has at soaking damage.