The supporting character I present to you today is, quite literally, a killer. At least in intention, though not in fact. He’s been my favorite supporting character since the first time I came across him--over twenty five years ago!
He’s not an obscure character, so I'll give you a chance to guess. His untimely death gave him eternal youth in our minds, but he’s only slightly younger than Dracula. He’s unique and timeless, but not undead. He’s irreverent—a fun-loving joker. He talks of dreams, he has unpredictable swings.
I believe he knew, at some level, that he would die young. He’s the embodiment of "carpe diem", in high contrast to the protagonist he supports so ably (a romantic given to writing poetry inspired in platonic love), and I believe the thirst of life of our character is driven by foreknowledge, at a subconscious level perhaps, that he would not live long.
His monologue is famous—pure magic of youth tinged with a fatality that grips the reader (or listener, for it was written to be spoken)—, as is his loyalty: unable to understand why his best friend will not fight the man who has insulted him, he draws his own sword against the threatener and dies. That thrust of sword under the arm of his best friend sets off the events that culminate in one of the literary world’s most poignant tragedies—so easily avoidable, had pride been less entrenched.
Did you guess Mercutio?