Riku is the name of a sumo wrestler who appears in The Ring of Sky.

The Ring of Sky

He is one of the sumo wrestlers hired to entertain Daimyo Kato, an unjust Samurai lord.

Jack and Akiko are captured by Kato's samurai and Kato forces Jack to fight Riku, a massive sumo wrestler, or else Akiko will be killed.

Jack is initialy beaten by Riku, who is a formidable oponent, but then understanding the rules, challenges him to another fight, best out of three. Jack fails to execute the cho-geri on him but still wins after felling Riku.