Nationality: Fire Nation
Age: Mai 15 - Zuko 16/17
Hair color: Black
Eye color: Mai Brown - Zuko Gold
Zuko and Mai's relationship didn't start until he joined his sister's side earning his return to the Fire Nation, but it was hinted that they had a crush on each other when they were little (confirmed by Azula). The first time they are seen kissing is during their set up "date" in the Earth Kingdom, and later on their way to the Fire Nation.
During a brief vacation to Ember Island they break up because of Zuko´s anger at himself but later make up, with Mai saying that he's the one she cares about the most. Before leaving to train the Avatar he leaves a goodbye letter to her, and later tells Sokka that he didn't want Mai to be seen as a traitor.
In the prison he says almost proudly that she doesn't need protection from a guard, and that he must help the Fire Nation. In order to help him and the rest escape, she betrays Azula and tells her that she loves Zuko more than she fears her (Azula). After the war is over and his coronation is about to start, she helps him get dressed and they renew their relationship through a passionate hug, and romantic kiss.
Source: Avatar Wiki [link]