Hmmm... I reckon it could be Sin.

A sight revered, yet with the sound of death, - Lots of people are in awe of Sin, but he kills them (I think I'm way off )
Found in the darkness, with other's waiting, - Um... it's dark when you fight (SPOILER)your aeons?
One of the few, who doesn't kill with it's breath, - Does Sin use a breath attack at any stage? This may be referring to Giga Gravitron, which Sin only uses when its mouth is fully open.
Many who tried, found themselves failing. - Many have completed the Final Summoning, but none really succeed in destroying Sin

Not knowing the truth, yet fearing the past, - Yuna and co. don't know the truth about the Final Aeon.
Yet you you prevail, the trio will help, - Jecht, Auron and Braska? You see bits and pieces of their pilgrimage at Zanarkand and through Jecht Spheres
But beware, though the enemies death comes fast, - Dunno. Yu-Yevon has not much HP perhaps?
But the strong become weak, just like that little whelp. - Yu-Yevon starts off strong (as Sin) but ends up weak (his final form)