Crosis was a Borg who was part of a rogue group of drones that gained individuality and fanatical emotion by following an entity called "The One," which in actuality was the evil android Lore, Data's twin.
In a bizarre scheme to lure Data to Lore's base in 2369, Crosis beamed aboard the Enterprise-D and let himself be captured. While in the brig, he conversed with Data and reminded the android how much pleasure he felt when he killed a Borg earlier, during an impossible display of emotion. Crosis slowly convinced Data that it was worth anything to feel an emotion again.
Following this meeting, Data stole a shuttlecraft from the Enterprise with Crosis in tow, and the two travelled through a subspace distortion toward the rogue Borg's base, where Data was reunited with Lore.