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The Fighting Style Kyokugen Ryū (Kyokugenryu) Karate of the Ryūko No Ken aka Art Of Fighting/Garou Densetsu aka Fatal Fury and King Of Fighters Fighting Game Universes is based on the real life Martial Art Kyokushin Karate created by Korean Karateka Choi Yeong-eui aka Mas Oyama.
The Character Takuma Sakazaki the creator of Kyokugen Ryū (Kyokugenryu) Karate and the father of both Ryo and Yuri Sakazaki is based on the creator of Kyokushin Karate Choi Yeong-eui aka Masatatsu Oyama aka Mas Oyama (Choi Baedal in Korean).
Kyokushin Karate is a Karate Style that emphasizes Full Contact Fighting. The only rules are that you're not allowed to Punch in the face or hit the groin and no eye gouging. Other than that it's Full Contact Karate all the way.