"Macnair . . . destroying dangerous beasts for the Ministry of Magic now, Wormtail tells me? You shall have better victims than that soon, Macnair. Lord Voldemort will provide...."
-- Voldemort to Walden Macnair in the Little Hangleton graveyard (GF33)
MacNair was sent by Voldemort to liaise with the giants; his attempts went somewhat better than Hagrid’s (OP20).
Macnair participated in both the Battle of the Department of Mysteries (OP38) and the Battle of Hogwarts (DH36). He didn’t seem particularly successful in either case, ending up with a wand jabbed into the eyehole of his mask (OP38) and crumpled on the ground of the Great Hall, having been thrown into a wall by Hagrid (DH36).
Walden is an Old English name meaning "child of the forest valley" (BabyNamespedia).
Macnair can be either a Scottish or Irish surname. It may derive from the Irish Gaelic Mac Iain Uidhir ("the son of sallow John"), the Perthshire name Mac an Oighre ("son of the heir"), the Gaelic Mac an fhuibhir, ("son of the Smith") or Ulster Mac an Mhaoir ("the son of the steward or keeper") (Internet Surname Database).
According to a (non-canon) planning chart for Order of the Phoenix, Macnair was the one visiting Bode at St. Mungo's on Christmas Eve (JKR). It is very likely that Macnair was the "friend" who brought Bode the Devil's Snare that killed him (OP25).