Guy Gardner, like John Stewart and Green Arrow is one of those comics characters who start their careers as trivia questions (cover of his first appearance by Gil Kane, all rights to current holder), then eventually rise to A or at least B-list status.
In that first story, Hal Jordan learns from the Guardians that Guy, a gym teacher, also met the qualifications (honest, fearless) to replace GL Abin Sur when he lay dying on Earth; Hal was closer so he got the nod. A Guardian device for viewing alternative futures shows that a later space adventure would have wound up killing Guy and Hal getting the lamp anyway. At the end of the story Hal meets Guy without telling any of this. Given writer John Broome liked bringing bit players back (Hal’s family appeared semi-regularly throughout the Silver Age), I’m sure we’d have seen him again.
When we do see him again in Green Lantern #87 it’s Denny O’Neil writing and things have changed. Guy knows about the Green Lanterns and that he’s Hal’s designated replacement if the latter buys it. In the opening of the story, though, Guy breaks his leg so Hal has to pick John Stewart as his backup instead. Later, in GL #116, Guy takes over for Hal briefly, but apparently dies. In reality he’s been caught by the Phantom Zone villains, who send him against Hal and Superman. Guy winds up in a coma.
And there he lay until Steve Englehart’s run on Green Lantern. Englehart had a flair for taking minor characters and turning them into hits, and Guy was one of them. During DC’s epic, continuity-rebooting Crisis on Infinite Earths, the Guardians mostly give up on defeating the bad guys. One who doesn’t agree revives Guy from his coma and sends him out to put things right (this was part of a tie-in arc in Green Lantern). As this Guardian’s plan is fatally flawed, Hal intervenes to stop Guy. Even so, Guy now has a ring of his own.
It’s not the same Guy, though. After years of helplessness, he’s angry, determined to make up for everything he missed, and a little pissed that an accident of geography stopped him from being Earth’s Green Lantern. He was loud, crass, obnoxious, willing to hit first and ask questions later and became popular as an anti-hero as a result. Some stories implied this resulted from brain damage, but nobody ever tried to cure him.
In the 1990s, his popularity led to his own book. Having been stripped of his ring, he found one of the villainous Sinestro’s yellow rings to replace it. At the time, the yellow ring charged itself from emerald energy (the idea they were fueled by fear came later) so when DC later wiped out all the Green Lanterns but newbie Kyle Rainer (Hal wasn’t wiped out but he implausibly turned evil because, 1990s!) Guy’s ring became useless. He operated for a while as a non-powered hero, but DC wanted more. Then-writer Beau Smith decided Guy was the result of a millennia-old ET program to create a super-warrior (as Brian Cronin recaps here). Unfortunately DC decided he should also have some sort of Power Ranger morphing ability and so it became (as noted at the link).
When Geoff Johns brought back Hal and the GLC, Guy lost his alien powers and found a new ring. He’s been written since as less brain-damaged thug and more tough and surly, a little like Ben Grimm. More recently he’s wound up as one of the leaders of the Red Lanterns. While I don’t think he’s been as successful as John Stewart, he’s been successful enough that he won’t be fading from view any time soon.