Ser Jorah Mormont is a formerly exiled Northern lord from Westeros previously living in Essos. He has sworn fealty to his fellow exile Daenerys Targaryen and was the first to help her adapt to life as a Khaleesi of the Dothraki.
Originally, Jorah was working as a spy for Varys, King Robert's spymaster in King's Landing, sending Varys information about the Targaryen exiles. But he grows to respect and admire Daenerys and begins serving her in earnest. He saves her from an assassination attempt, stops sending reports to Varys and ignores an official royal pardon that would allow him to return home to Westeros. He develops strong and unrequited feelings for her.
When Daenerys learns of his past spying, he is dismissed from her service and ordered to leave her presence. He later captures Tyrion Lannister in the hope of winning his way back to her, an endeavor that leads to him contracting the deadly greyscale. After he helps save her life from both the Sons of the Harpy and the Dothraki, Daenerys accepts him back to her service, and orders him to come back to her after he finds a cure to his disease, which he eventually does at the hands of Citadel acolyte Samwell Tarly at Oldtown.
(source GOT wiki)