Picture(~16 year old):
Expanding and adding on to the information that fullblue provided earlier:
Japanese Pronunciation: (Ei) Sei Kyou
Chinese Pronunciation: (Ying) Cheng Jiao
Though his surname was not mentioned in the manga, it was assumed that he has the same surname of “Ying” as Zheng as they are brothers who share the same father(i.e. King ZhuangXiang).
2.Activities in History
As mentioned by fullblue, he will rebel again in the future of the manga, or 8th year of Ying Zheng’s rule to be exact.
From ShiJi: Annals of Qin Shi Huang
Original Text: 八年，王弟长安君成蟜将军击赵，反，死屯留，军吏皆斩死，迁其民于临洮。
Translation:8th year(of Ying Zheng’s rule), the king’s brother, Lord Chang An, General Cheng Jiao, attacked the state of Zhao. He rebelled in Tun Liu and his subordinates were all executed. The people from Tun Liu were also exiled to Lin Tao.
This entry of ShiJi clearly stated that Cheng Jiao revolted in the 8th year of Ying Zheng’s rule. While there was a rebellion recorded in the first year of Zheng’s reign, there was no mention of who started the rebellion or why. Hara sensei probably filled in this blank with Seikyo’s rebellion in the first arc of the manga. According to baidu(chinese equivalent of wikipedia) and fullblue, Cheng Jiao escaped to Zhao after the failed rebellion and was given the land of Yu Rao by King DaoXiang of Zhao though i can’t really find any historical records on that at the moment.
3. Interesting Stuff
According to the official Kingdom guidebook(picture above), Cheng Jiao believes that Man were all born evil, which was initially suggested by Xun Zi(http://en.wikipedia.org/wiki/Xunzi) and that was also the opposite of Confucius teachings which suggested that Man were all born to be kind. We might never find out whether Cheng Jiao thinks that way in actual history but Li Si, the student of Xun Zi and future prime minister of Qin, convinced Ying Zheng to accept Xun Zi’s thinking and that was part of the reason why Ying Zheng enforced such harsh laws to tame the “evil” in his people.