Drake is a renegade who Will and Chester encounter while in the Deeps. He was a teloelectronic scientist, creating scopes, night vision technology, and more. The Styx initially captured him in an attempt to get him to give them scopes and other things. He's been a significant character in the plot of the series. While in the Deeps, he encountered Elliot.
In Tunnels, Drake has not yet been introduced.
Drake first encountered Will and Chester in Deeper, around halfway through the book. While Will was still having trouble dealing with Cal's death, they were observing what appeared to be some sort of Styx event when Drake and Elliot came, and essentially kidnapped them.