Zuko is one of the main characters in the animated series, Avatar: The Last Airbender.
Zuko is a highly-skilled firebender and was Crown Prince of the Fire Nation, but early in his life, he was banished from his home by his own father, Fire Lord Ozai, for speaking out of turn and refusing to duel him afterwards. During said duel, the left side of his face was burned by Ozai, leaving him with a noticeable scar. Soon after he was banished, Zuko made it his mission to capture the Avatar, who had disappeared for almost 100 years at the time, because such an accomplishment would put an end to his banishment and restore his honor.
During his long journey and after much turmoil, Zuko eventually realized that his father was an evil tyrant that needed to be stopped, so he decided to leave and join the Avatar instead. After proving himself to be a changed man to Avatar Aang, he became his firebending mentor and a member of Team Avatar. This team soon stopped Ozai and put an end to the Fire Nation's warmongering ways, not only restoring Zuko's honor, but the honor of his family and home. Because of his efforts and due to Ozai being imprisoned, he became the new Fire Lord and started a new era of love and peace.
Zuko's adventures continued far beyond the original animated series. He appeared in many graphic novels that told of his struggles of being the new Fire Lord. He also made several appearances in the sequel series, The Legend of Korra, which is set roughly 70 years after the events of the original. No longer the Fire Lord after he abdicated to throne which was was then passed to his daughter Izumi, he became an ambassador of peace. Even at the age of 88, Zuko still maintained respectable combat ability, but his speed was still significantly reduced due to his advanced age.
Throughout the original Avatar series, Zuko's appearance changed several times, including both his hairdo and his clothing. His physique, however, was somewhat more consistent, being lean yet muscular throughout most of the series. The definition of his muscles usually had slight changes between episodes, depending on what animation studio and artists were involved.