He comes with you to the next level in the Nevada Desert, but you two split up on the basis that this fellow "works better alone."
You couldn't believe my fanboy craze when I first saw the character jump in randomly. Throughout the next level you communicate through a headset with him, and he's known only as an "NSA Agent."
Partly because I saw the character (and because the level is set at night), I went and customized my character to look exactly like a Splinter Cell. Very light armour, balaclava, goggles...anyway, really, really cool. Anybody else here play rainbow six?