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Muramasa can take over Ryujin Jakka, I don't see why it couldn't do the same to KS. Clearly even top tier Shinigami can't stop it. In addition, Koga's own ability does the same thing KS does. Aizen is probably a better Shinigami, but he actually gets out-haxxed here.
Depends if Koga lets Aizen use KS.
Hax vs Hax and I see Aizen win that scenario.
If the opposite happens (Koga uses Muramasa against Aizen), then Koga probably wins this.
If Koga has Muramasa he should win this conjugated with his skills. Both are pretty hax.