For those unacquainted with the television series, Due South, here is a very quickie background.
Constable Benton Fraser (Paul Gross), a member of the R.C.M.P. Fraser came to Chicago in pursuit of his father's killers. In the process, he uncovered corruption in the Mounties and embarrassed several high ranking Canadian officials. He was exiled to Chicago, obstensibly assigned to serve at the Canadian Consulate.
One of Fraser's best friends is Diefenbaker (Draco), a wolf cross, who saved his life when he fell through an ice flow. Dief is suppose to be deaf, but seldom acts like it. He's also a big junk food junkie, much to Ben Fraser's disgust.
In the process of Fraser's man-hunt, he became best friends with Ray Vecchio (David Marciano), Chicago detective. Vecchio was a good cop in a downward slide, until he met Fraser. Both are stronger for their friendship. Fraser frequently assisted Vecchio in his cases.
In the beginning of the third season, Ray Vecchio went under cover in the mob leaving Stan "Ray" Kowalski (Callum Keith Rennie) covering for him in his duties as cop. Ray Kowalski inherited Ray Vecchio's life, including his sister, Frannie (Ramona Milano) and his best friend, Ben Fraser.
Christina Cox appeared in two episodes (Free Willie and The Vault) of Due South as Morgan, a ruthless thief. Both episodes were with the original Ray Vecchio, not the "new" one, Ray Kowalski.