Charlie was the bass player for the band DriveShaft, a group he started with his brother Liam. The group achieved some level of fame, but broke up at the height of their popularity. During his time with DriveShaft, Charlie became addicted to heroin.

Sometime later, Charlie flew to Australia in a failed attempt to convince Liam to reunite the band. Presumably, he got on the doomed flight 815 soon after this conversation.

Locke tells Charlie he knows his secret. He convinces Charlie to give up drugs, liberally applying metaphor and Island appeal. In the end, Charlie gets back his guitar in exchang for his "sacrifice" to the Island. But he still comes back to Locke, and asks Locke 3 times for his drugs back. The third time, Locke thinks he has given in, but Charlie tosses the heroin into a fire.