"Dave was an intelligent mastermind. I don't know if he died or if he's alive, but what I know is that he's a fuckin' brainiac.
Dave Miller is an unseen character mentioned by Sam in the chapter "The Wasteland Army". He is a major resistance member from the Resistance Territory in Pre-War California. It is hinted that he was majorly injured in a incident known as "Bravo", which was a raid by the Combine.
It is possible that he was killed by the Combine during the "Bravo" raid. It is not known, but it can be suggested due to the fact that Sam said that Dave had major injuries.
Chapter One - "The Wasteland Army" (mentioned)