Season 1 and Season 2.
David Giuntoli as Nick Burkhardt (Seasons 1-2)
Nick is a homicide detective in Portland, Oregon, whose abilities of perception and deduction have helped him greatly in his job. He soon discovers that his talents lie in his bloodline; he's descended from an elite line of criminal profilers known as "Grimms" who have the ability to sense and identify supernatural creatures ("Wesen") who masquerade as human. Grimms are a long line of humans who serve as a secret police force, quietly protecting humans from any rogue Wesen who seek to do them harm. Thus, during the course of his "human world" police work he often comes across culprits from the "Grimm world" whom the rest of humanity sees as human. Juggling his duties in both worlds proves troublesome, especially when trying to keep his Grimm status a secret from his human co-workers and friends.
Bitsie Tulloch as Juliette Silverton (Seasons 1-2)
Nick's loving, calm live-in girlfriend, Juliette is a veterinarian who is unaware of his Grimm status -- and thus also unaware of the danger she's in from Wesen who are targeting Nick. She does, however, sense that Nick is hiding something from her and thus when he proposes to her, she declines.
Russell Hornsby as Hank Griffin (Seasons 1-2)
Hank is Nick's police partner in Homicide and his best friend. He has a likeable personality, as evidenced by his being married several times in the past. Although he is unaware of Nick's secret life, he knows that Nick has an uncanny perceptive ability. As he works more and more cases involving Wesen disguised as humans, he becomes increasingly aware that something strange is going on, and he comes to develop an acute case of paranoia.
Silas Weir Mitchell as Monroe (Seasons 1-2)
Monroe is a Wesen whose creature form (a "Wieder Blutbad") is werewolf-ish in nature. He was initially a suspect in one of Nick's cases but now has become an unofficial partner who helps Nick solve crimes in the Grimm world by sharing his expertise regarding the various species of Wesen. Although he is a good, loyal ally, Monroe is naturally a bit wary of Grimms, and Nick's demands often strain his nerves, causing frequent sarcastic complaints.
Sasha Roiz as Captain Sean Renard (Seasons 1-2)
Captain Renard is Nick's boss at work and is secretly aware of Nick's Grimm status and of the Wesens. In fact, he appears to be royalty of some sort in the Grimm world, as some other Wesen refer to him as "your highness," although his exact nature (Human? Wessen?) is unclear, as is his intentions toward Nick. At times he appears to be a nemesis, while at others he supports Nick -- particularly in the human world, where he is a respected police officer.
Reggie Lee as Sergeant Wu (Seasons 1-2)
Wu is a colleague and friend of Nick and Hank and works closely with Renard. He's a helpful, capable officer with a good sense of humor.
Bree Turner as Rosalee Calvert (Seasons 1-2)
Rosalee is a fox-like Wesen known as a "Fuchsbau" who sometimes helps Nick and Monroe in their investigations, aided by her knowledge of medicines. She is a converted bad girl who's on the right track, running a spice shop catering to Wesens. Rosalee is a romantic interest for Monroe.