Liú Zhèng (劉正 Ryūshō) is a character mentioned in the original Hokuto no Ken one-shot. He was the 21st Hokuto Shin Ken successor who trained Kenshiro Kasumi's father, who in turn trained Kenshiro Kasumi himself.
It should be noted that in Souten no Ken, Ramon Kasumi (who later becomes Ryūken, Kenshiro's adoptive father) was actually trained by his father Tesshin Kasumi, who is the 61st successor of Hokuto Shinken, not the 21st.
In the pilot, Hokuto Shin Ken is referred to as a Liú Family Secret Tradition (劉家秘伝 Ryūke Hiden). This "Liú Family" concept was apparently dropped for the regular manga, but it strangely resurfaces during Kenshiro's fight with Uighur and is never mentioned again. It is established in Souten no Ken that the Liú Family fist is actually Hokuto Ryū Ken.