Colby Granger joins the LA FBI team at the very beginning of Season 2, and rubs David Sinclair the wrong way from day one. Fresh out of the military, Colby has a black-and-white worldview and an apparent belief that sufficient force can always get the job done.
Colby, however, proves to be far more than just an Idaho boy who found his calling in the army, and over the course of the series he is revealed to be first a Chinese spy and then a seeming double agent. His multiple identities cause immense friction within the unit, and are at least a factor in Megan Reeves' decision to find a lifestyle that doesn't get so fraught and confusing.
By the fifth season at latest Colby has repaired his relationship with David and the two function once more as a dyad, although David's promotion at the end of the series does compel Colby to partner up elsewhere.
Colby is friends with Roger Bloom, the relationship even developing into a near-filial one. Whilst he begins as typically ex-military, Colby comes to see the value of the other approaches used by the team, in time learning to respect the 'geek input' of Charlie, and to recognize the merits of Don's instincts.