Howard Keel said of Russ Tamblyn after making Seven Brides For Seven Brothers : "Wherever he was, you couldn't take your eyes off him." I think it's safe to say that the same thing could be said when Riff is on screen. Arthur Laurents described the character as "glowing, driving, intelligent, slight wacky" and the Riff you see on on-screen is that and more. His joie de vivre from being "Number One, to sail, to hold the sky" is palpable especially in the Prologue.
He's also the surrogate little brother to Tony, he feels abandoned by his big brother and doesn't understand how or why anyone - especially Tony - wouldn't want to be a Jet. He may think he "knows Tony, like I know me" but this crazy notion of quitting the Jets and getting a job is beyond him. So when the moment comes to finally crush the Sharks all he can think of is getting Tony back, so he can be literally at his back like before, when he challenges Bernardo. And this will be the act that will doom Riff, Bernardo and Tony because Riff cannot abide seeing Tony treated the way Bernardo and the Sharks do at the rumble; he steps in and literally offers up his life to save Tony from humiliation.
At the same time, he's very much a villain; when crossed he gets a cold, mean eyed stare and is capable of exploding into deadly violence. He proves himself a ferocious opponent when he punches Bernardo - look how far Bernardo flies backwards - and as he gets the better of Bernardo just before Tony pulls him off.
Riff has all the trappings of the movie tough guy, including the sassy girlfriend, but that natural leadership and drive has no place to go. He's a bright natural leader who the Jets automatically look to for leadership but it's all wasted in a world that's "lousy". And worse, deadly for young men who can't see beyond a few blocks of pavement.
And to quote Barry Monush in West Side Story : Music On Film :
That there was a feeling of loss when he exited the picture long before the wrap-up was a testament to how much strength he had brought to it.