Been thinking about this for awhile but it seems Treble really doesn't have any practical use in this. Of course maybe this may suffice thanks to some of the suggestions given here and combining them together, so credit should go to them more...
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Treble
Item Status: Assault
Duration: 30.00 seconds
Item appearance: Rare ((uncommon))
Info: I'd say the function of Treble should work similar to how it was implemented in Power Fighters, though with some add ons that make's him worth hunting down.
First off when using the Treble item, he should normally follow you around and attack any enemies near by with his mouth cannon. However your only limited to using your Mega Buster in this state. Since Treble is giving you covering fire, the target will have no choice but to either do some creative dodging or flee from the plethora of bullets.
Sounds crappy and pointless? Thats just the neutral use of this weapon mortal...
What if your opponent is too jump happy or hyper in movement? Treble should eliminate that. By pressing the "use" key (( the command key that you use for opening doors or pressing buttons in normal doom stages)), Treble will pounce on the enemy in your cross-hair.
If contact is made, then the player's speed is cut and jumping is disabled until you shake him off ((by pressing the jump button enough times of course. I'd say about 15 presses will work)). The pounce hold should do about 2 points of damage on contact, but only when he hits you, not when holding you down for the duration of the time. The pounce command can be used multiple times
if contact failed, however if the player was pounced and then successfully shakes Treble off ((or was killed failing to do so)) then you cannot use pounce again considering the time limit he has.===============================================
Your call. Yay or nay man

EDIT ((Oct 1 2010)): Since the Pounce Hold Idea turned out to be a hell of a lot of coding involved, half of the idea as been cut down to make it a more code friendly idea. The basic pounce Idea was suggest by Korby and possibly a few others in this topic so yeah. Like I said, I was just mixing stuff together to make Treble more useful.