Courier and Gambit (Remy LeBeau) have a decent working relationship, but not much more. Courier seems to resent the way his powers and job seem to let others use him, but he's not strong enough to take a stand and change his fortunes. In a trip with Gambit back in time to the late 19th Century, Jake pulled off a stunning use of power and morphed himself into a woman, "Jacqueline," in order to infiltrate Dr. Nathan Milbury's obstetrics clinic. Milbury was of course, none other than Mister Sinister, who upon realizing that Jacqueline was not all she appeared to be, injected her with a compound that reverted Courier to a liquid state.
Courier was imprisoned in a bubble until Gambit agreed to give Mr. Sinister a piece of the human liquid in exchange for stabilizing his friend. Unfortunately, Gambit thought that Jake/Jackie would be able to kill the separated piece afterwards, as he had done before. Unfortunately, Sinister's stabilization of "Jackie" removed his mutant powers, and Sinister was able to co-opt the morphing capabilities of Courier's cells into his own, giving him the complete control over his cells that Gambit had seen so often in his own time. Jake's lack of powers, also meant he was stuck in a woman's body. Courier still worked for the New Son, but was realizing that his methods were leading to madness and he left with Gambit when he and the X-Men attacked the New Son's citadel. Soon after, Courier helped Gambit break into the Pentagon by posing as Professor X in order to get more information on the mysterious "Black Womb." It is currently unknown if, um, he has kept his powers after M-Day and what affect they might have on his body.
Courier has the ability to control every molecule of his body, allowing him to heal wounds and prevent infections, among other things. He can even survive bullets to the head and regrow limbs. However, it appears that he doesn't grow cells back so fast, so he may simply rearrange his cells for the new limbs. This may be why he is now 5-foot-eight when he used to be 6-foot-two.
His ability makes him the perfect messenger, as his father and his employer the New Son would have to agree.