He is the Braavosi commander of the Second Sons, a sellsword company. Despite his superficial friendliness, Mero is a serial rapist with a cruel reputation, and under his leadership, the company is rarely hired by anyone.
In the show portrayed by Mark Killeen
In the third book, Mero commands the Second Sons and are invited by Daenerys Targaryen. He was hired by the Yunkai forces. In the show, he is a joint commander with Prendahl na Ghezn and Daario Naharis, who are members of the Stormcrows in the books. During the meeting, he insults her several times, with Mero refusing to switch to Daenerys' side as part of her plan to liberate Yunkai, and Daenerys refusing to surrender. He accepts a barrel of wine to represent Dany's wish to hire the company, and leaves.
After the Second Sons depart, Barristan Selmy (disguised as Arstan Whitebeard in the books) and Jorah Mormont agree that Mero has an "evil reputation" throughout the Free Cities, and have lead the company to become nearly as bad as the Brave Companions. This is the reason why nobody would hire them.
During the Battle of Yunkai in the books, Mero shaves his beard and hides amongst the freedmen that follow Daenerys. He attempts to murder her but is ripped apart by a mob in the camps.
In the show, Mero dies in a different manner, since the Second Sons and the Stormcrows are basically composite (the Stormcrows do appear in the show but as nameless sellswords hired by Stannis Baratheon). He, alongside Prendahl and Daario, decide to kill Daenerys inside a brothel with a Yunkish bedslave. They let the bedslave hand out three coins and random, and the Braavosi is given to Daario, leaving him the task of assassinating Daenerys. Daario kills the other two instead and presents their heads to her.