All you really need to know to know about Guardian:
Canadian James MacDonald Hudson helped his government create Department H, a research and development branch that allowed him to create his Guardian battlesuit as well as to train and develop superheroes. Alpha Flight was its first tier of champions, with Beta Flight and Gamma Flight below it. When the government shut down the program, Guardian and Alpha Flight nevertheless continued to fight the good fight on their own.
What you may or may not need (or want) to know to know about Guardian:
Mac died (the first time) when his former co-worker created the team of Omega Flight to take revenge against him and his battlesuit exploded.
BUT in fact he was not dead, just thrown backwards through time and space and captured by aliens who revived him by turning him into a cybnertic composite with the leftovers of his battlesuit and left him buried in the Earth. His former company discovered him and used him until Alpha Flight rescued him. But the aliens all along had wanted to use New-Mac as a weapon against Galactus, calling Alpha Flight to a distant galaxy. Successful in thwarting Galactus, Mac apparently sacrificed himself (again) in order to return his teammates back to Earth.
BUT in fact he did not die; he was just shunted to into "null space" where he was later discovered by Alpha Flight's archenemy, the Master, who bent Mac to his will and sent him against Alpha Flight. It was Heather`s love for him that broke him free and help defeat the Master. They would have lived happily ever after except, years later, a new Department H tried to force the New-new-Mac into performing dubiously criminal experiments. The New-Department stripped Mac of his mind and shot him into space.
BUT a 19-year old version of Mac appeared in cahoots with this New-Department H and started a New-Alpha Flight. Turned out this guy was a clone, and the Old-new-new-Mac was found and revived by his teammate Sasquatch (the original one, not the New-Sasquatch that was on the New-Alpha team at the time) so that together they could re-form a New-new-Alpha Flight to confront Department H. The two versions of Alpha Flight soon reconciled and joined together as one. It was a confusing time. Thankfully, Clone-Mac and Original-Flavor-Mac were soon kidnapped by evil A.I.M. scientists who wanted to study them both. Alpha Flight and Wolverine freed them, but Clone-Mac was killed during the escape.
BUT another alternate-version of Mac (along with alt-versions Heather and original Alphans) were brought into the present after a New-new-new-Alpha Flight (yes, even I am starting to lose track at this point) messed around with a time-warping villain. We're pretty sure that these guys were all killed off later by the emergence of some guy called the Collective (who was created by absorbing all the left-over energies when the world's mutants… you know what, never mind about that. I'm not going to tangent into all of that.)
BUT Mac and Alpha Flight reappeared, most likely because the Real-Mac (can I even say that anymore?), Heather, and the gang had previously left Earth at one point to make sure some alien eggs that threatened to invest the planet would be cared for, and they came back after the whole Collective thing.
OR perhaps it's because Mac, Heather, and the gang were briefly released from death when the Chaos King/Japanese god Mikoboshi defeated death in order to collapse reality during the Chaos War, and they just stuck around.
EITHER WAY, Mac Hudson is currently starring in Alpha Flight's latest comic series, together with Vindicator, Sasquatch, Northstar (sorta), Marrina, Aurora, and Snowbird.