The best sword yielders in Usagi's world are obviously skilled, but they've also had years of training practice. We've seen Usagi
train to become one of the best samurais in the kingdom, we saw Gen
being trained by his father, we've had the opportunity to see Tomoe
training since she was a kitty.
However, for one person in this particular universe, the ability to fight with swords came by as naturally as breathing. Her name is Inazuma, "the Lightning". She took on Gen and would have killed him if it weren't for a slip. After that, she fought both him and his fellow bounty hunter Stray Dog simultaneously and almost won, having to escape at the end.
We saw her origin early in just her second appearance, since the desire for revenge was her initial motivation.( Trigger warning for domestic abuse )
This story was collected in Book 10: The Brink of Life and Death
and Usagi Yojimbo Saga vol. 1
Inazuma is my definition of "badass" in the Usagi Yojimbo universe, more than Tomoe, Katsuichi-sensei and Usagi himself. I've read in some places that Sakai himself acknowledges her as the best fighter in their universe, but never saw a direct a quote from him. Anyway, I do not dispute that affirmation.
And to end in a typical Scans Daily vibe - I think she would be the most attractive woman of the series if they were real people instead of anthropomorphic animals.
Tomorrow: Sasuké the demon queller.