Fenn Rau is the leader of the Protectors of Concord Dawn, voiced by Kevin McKidd in Star Wars Rebels. StarWars.com introduces him as a "veteran of the Clone Wars and a respected warrior" who "led the Protectors of Concord Dawn with a combination of honor and ruthlessness", noting that he served as a flight instructor during the war and fought at the Battle of Mygeeto.
The arrangement
The Protectors control a hyperspace corridor and sell safe passage through it. Under the Empire that becomes a working relationship: Rau enforces Imperial interests in his own airspace in exchange for being left alone to run it. He is not an Imperial and would be insulted by the suggestion; he is a man who has made a deal.
What changes his mind
Captured by the rebels, he is in their custody when the Empire destroys his own forces — the clearest possible demonstration that the deal was never mutual. The databank's summary is exact: "Witnessing Imperial destruction of his own forces led him to ultimately join the Rebellion."
Elder statesman
Afterwards he functions as the Mandalorian material's institutional memory, the one character present who fought in the Clone Wars and can explain what the clans used to be. He is also, usefully, the counterweight to Gar Saxon: two Mandalorians who both collaborated, one of whom stopped.