Baelic is a recurring character in the third, fifth, sixth, and seventh seasons. He is a renowned sellsword originally from the Summer Isles, and is the younger brother of Maester Aderic of Brightbank.
He was formerly an associate of the Brotherhood Without Banners (but he was not an official member of the group) and later became travelling partners with Arya Stark and Sandor "The Hound" Clegane. Alongside the both of them, Baelic witnesses the final stages of the Red Wedding, and was believed to have been killed when he buys time for Arya and the Hound escape.
Baelic later resurfaces as the Captain of his own sellsword company, the Midnight Sons, having survived the Red Wedding. He later offers his services to Carmine Grayburn, adding his sellswords to Grayburn's Legion, and rises to become one of Carmine's chief lieutenants. However, after Carmine is killed by the forces of House Bolton, Baelic and a third of his Midnight Sons deserted the rest of Grayburn's Legion for unknown reasons. It is later revealed that Carmine had, in fact, faked his death and discreetly traveled across the Narrow Sea to Meereen to gain an audience with Daenerys Targaryen, and Baelic went with him, taking the name Darksteel, while Carmine became known as Stormbringer.
Salladhor Saan is well acquainted with Baelic and claims to have worked with him on numerous occasions.