Segata Sanshiro was a fictional character and parody of Sugata Sanshiro, a legendary judo fighter. He appeared in two dozen commercials by SEGA to advertise the Sega Saturn in Japan between 1997 and 1998.
Sanshiro was a Judo master that tracked down and beat up people who were not playing the Sega Saturn. His name sounds similar to the phrase "Sega Saturn shiro!" meaning roughly "You must play Sega Saturn!"
The ads are over the top, but are funny and entertaining. Watch some of them here.
Hmm... not quite how I should be preparing for JLPT, is it?