"The style is very similar to Saint Seiya", then it should be this:
"The True Colors of Hot Fist"
Alias: Dragon Phoenix Boxing King, Boxing Champion Genesis
Author: Masami Kurumada
Plot:
In order to fulfill the legacy of his father who died of illness before the championship match, Koro Ryu sets his goal to be a boxer. Because Er and her sister Xiaoju, who was a coach, could not bear the harsh treatment of their stepfather, they left their hometown in Yamaguchi and came to Tokyo, where they worked as a helper at Kanako Sanjo's house and studied at Seika Academy. Kenzaki Jun, the president of Seika Academy's boxing club, made Ryuji join the club as a condition, and the two had a practice match. Ryuji, who was being strongly attacked by his colleague, defeated Kenzaki with his left jab.
Later, the stepfather found the Sanjo home, and the two were forced to leave and live in the Omura Boxing Gym. Long Er, who is in middle school, accepted Xiao Ju's suggestion and wore gravity wrist braces and gravity ankle braces. As a result, his boxing strength improved by leaps and bounds!
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