I'm usually a frankenstein Ninja/Recon/Gap-filling support.
I'm usually sneaking around, being stealthy, going the opposite way of the rest of the group, and seizing oppurtunities to attack an unsuspecting enemy from behind, while at the same time keeping my eye on the group, and sniping and what not when they need it, or dashing in to do some other job when it's called for. For example, in CTF on Halo, I'll sit back at the base, guarding the flag while everyone else goes on the offensive, only to swoop in and defend the flag bearer when they get close, or jump in and grab the flag if they fall. Otherwise I'm the guy in the banshee, giving air support, or the gunner on the warthog, wiping out the other team as we drive by. Basically, I'm there doing whatever needs to be done when it needs to be done, and then I switch back to being on my own and picking out what needs a-doing.