She was a young Immortal, who was found by Axel Whittaker. He used her like several others before to lure older Immortals to him, so he could take their heads from ambush. Axel tried to take the head of Vlad Dobrinski with Rachel's help, but Dobrinski didn't fallinto the trap. He beheaded Rachel instead.
Note: Rachel McClelland did not appear in Highlander: The Series, but was listed in the Watcher Chronicles.