Sasaki Kojiro was famous for being somewhat
vain about his admittedly
handsome appearance.
Women loved him, and he prided himself on his beautiful
hair. In an early encounter, Musashi sliced off Sasaki's
topknot. Perhaps this
humiliation started their
rivalry--they had more than one
duel. They're a famous pair of
rivals, and an
allusion to them represents worthy rivals in
Japanese culture.
The character
Ukyo Tachibana in
Samurai Shodown is loosely based on Sasaki.