Sumo is quintessentially Japanese. The wrestler is the ideal Japanese man: showing great strength, personal discipline and honor. For centuries, foreigners (gaijin) participated. Then, little by little, the door opened. The pioneer was Takamiyama Daigoro, a Hawaiian. He was the first to break into the upper ranks, and the first to have his own sumo stable, and did some foreign recruiting.
Konishiki (born Saleva'a Fualui Atisano) was a Hawaiian-born Japanese-Samoan wrestler. Takamiyama recruited him, and Konishiki was the first gaijin to reach Ozeki (champion) status. (He retired in 1997. I hear he now hosts a TV show and is a DJ. Kind of a cool life. Check out the link for more info.)
Akebono Taro (born Chad Rowan.) He was a Hawaiian, too, and dominated the sport (along with his Japanese rival Takanohana) throughout the 1990s. He had a longstanding and strong career as Yokozuna (grand champion). Like Takamiyama, he was honored to be allowed to join the JSA (Japan Sumo Association) and worked as a coach in the Azumazeki stable. In fact, he trained Asashoryu, the recently departed Mongolian Yokozuna (grand champion).
I guess I never thought of Sumo (also referred to as Ozumo, which puts the honorific prefix of the O in front of sumo) as being populated by anyone but Japanese men. But now it seems the JSA has had to start putting caps on the number of foreigners a stable can recruit. Like a lot of Japanese things, the Japanese like sumo to remain "unique" in the way only Japanese things can be unique.