Sean Renard is Captain of the Portland Oregon Police and thus Nick Burkhardt's boss, he is also of Royal, and is thus feared by the Supernatural world, something he's not afraid to use to his advantage.
Renard is the son of King Frederick Renard and one of his Hexenbiest mistresses Elizabeth Lascelles, making him the half royal (illegitimately) and half Hexenbiest, as such he spent much of his childhood on the run trying to escape from the queen (his Father's wife) attempts to have him and his mother killed. Somehow he ended up in Portland and rose to the rank of Captain.
Early on its revealed Renard has plans Detective Nick Burkhardt who he's fully aware is a Grimm however Nick isn't (initially) aware of Captain Renard's true intentions or identity.
Originally he seemed to be the main antagonist of the series, as his first acts are to have Nick's aunt (another Grimm) Marie Kessler assassinated, as wells as controlling Adalind Schade's efforts to try and steal Nick's Grimm key and keep Nick under his control. However as time went on Renard proved to be a much more morally grey figure. It was revealed he is a supporter of the resistance, and works to undermine and stop his family from regaining there power over the world. Particularly against his own older half brother Eric Renard.
Following Nick discovering the truth, Renard was able to convince Nick to hear him out, and the two men forged an alliance together (although Nick is still somewhat distrusting knowing Renard plays a lot different games), to cement Renard's good intentions he gave back Nick the key which he had stolen from him. Since then two of them agreed to work together to help maintain peace and order in Portland.
In Season 3, Renard has Eric killed, after Eric tries to kidnap and enslave Nick Burkhardt but later discovers he's accidentally fathered a daughter with Adalind. While consolidating and uniting the other resistance leaders, he begins working to keep her safe from his ruthless cousin Viktor Beckendorf who has replaced Eric as family Prince.