Yeo-Woon Cheon

Yeo-Woon Cheon was a member of the Cheon Family and was the son of a female servant who worked at the Lord's chamber and was not from the Six Clans. Although considered as an illegitimate son of the Lord due to a servant mother, he was still part of the Cheon family and he had the right to the throne but had an extremely low chance without support. He doesn't learn any martial arts in his early 15 years due to his oath with the Lord's wives from Six Clans. After entering the Demonic Academy, only then did he learn Martial Arts.

(Source: nano-mashine.fandom.com)