Choosing one Green Lantern was hard, so I’ll pick the most complex of them, Guy Gardner.
Origin: Guy was born in Baltimore with an abusive, drunk dad who only respected Guy’s cop brother, Mace. This led to Guy becoming a juvenile delinquent after no longer trying to respect his dad. Mace manages to turn Guy on the right path at 18, but Mace is revealed to be a dirty cop. Abin Sur crashes to Earth and his ring has to choose between Hal and Guy, but Hal was closest. Guy is left as the backup Green Lantern of Earth, a thing he hates very much.
Life: Guy’s life really is just a horrible series of tragedies. He is put into a coma soon after becoming a Green Lantern, so he is replaced by John Stewart. He wakes up and joins the Justice League International, where his brash personality conflicts with Batman. This is all culminates in Guy getting knocked out in one punch by Batman, which he is still made fun of for. He does start a relationship with fellow member Ice, which has been an on and off thing between them. He is currently residing in the Red Lantern Corps of the New 52.
Why he’s on the list: Between the five human lanterns, Guy has had the most changes happen to him. At one point he was revealed to be a Vuldarian, an alien race with the power to make weapons. He went by the name Warrior for a long time, but this was eventually just forgotten. He also wielded Sinestro’s ring for a while after his coma. Now he leads the Red Lanterns, as one of the few who can make energy constructs. His partnership with Kyle Rayner is also a very important part of his character. As stated by Hal, Guy’s ring glows because he is so eager to use his powers and his constructs are some of the strongest.
Recommended reading: Guy Gardner (Vol 1.), Green Lantern Corps, Justice League International (Vol 1.)