He is a Dothraki Khal.
He is first introduced sitting with his wives and bloodriders in his tent as Daenerys Targaryen is brought as a gift for him by two of his warriors. Impressed by her beauty, he circles her, opining that seeing a beautiful woman naked for the first time is the best thing in the world, however, his blood riders casually disagree, listing other great things, like killing a rival khal and enslaving conquered people. Moro, obviously mildly irritated by their flippancy, declares that seeing a beautiful woman naked for the first time is one of the five best things in the world and makes it clear that it's the end of the discussion.
As he is about to rip Daenerys's dress off to see her naked body, she firmly tells him to not touch her in Dothraki, surprising everyone as they were unaware she spoke their language. Daenerys then declares her identity, listing off her many titles. Moro, however, simply laughs in response, stating that he will still bed her and she will give him a son. Daenerys firmly turns him down, declaring that she will not bear him or anyone else children. Intrigued, Moro states that he likes that she has spirit, but his obvious lustful intentions are immediately dashed when she reveals that she is Khal Drogo's widow. Moro humbly apologizes for his behavior, for it is forbidden in Dothraki tradition to bed a Khal's widow. He then cuts her free of her restraints and gives his word that nobody will harm her. He refuses to bring her back to Meereen, however, as the same laws that protect Daenerys from molestation require that she be brought to Vaes Dothrak to join the dosh khaleen.
Moro later presides over the khalar vezhven. Moro advocated for Daenerys to join the dosh khaleen, while some of his rivals suggest handing her over to the Wise Masters, who have apparently offered 10,000 horses in exchange for her. Moro's conciliatory attitude ends, however, when Daenerys declares that none of the Khals are fit to lead the Dothraki, but she is. Moro threatens to have the khals gang-rape Daenerys for her insolence; in response, Dany overturns the braziers, setting the entire Temple ablaze. Moro is one of the few to make it to the door of the Temple, only to find the doors barred from the outside. Daenerys overturns the final brazier, killing Moro in the resulting conflagration.
Moro led the Dothraki horde that found Dany after she escaped the chaos at the Fighting Pit of Daznak. On learning that she was Drogo's widow, he commanded she be brought to the Temple of the Dosh Khaleen, in accordance with Dothraki tradition. After he insulted and underestimated Dany, she burned him alive - along with the rest of the Dothraki leaders.