A Guardian's Duty, A Friend's Honor
Knuckles had long since accepted that his life was to remain a lonely and isolated one. Being the only truly intelligent creature on his island in the sky, guarding the Master Emerald as was his duty, he fully expected to spend the remainder of his years with little to zero contact with the world down below. But then outsiders came crashing onto his island. The first, a large human claiming to be trying to protect the island. The others carried the seven Chaos Emeralds, and one bearing an incredibly strong connection to them.
Knuckles ended up siding with the first outsider, a little gullible when it came to other beings as well as a little jealous of the other's connection, though he would never admit it. So he attacked the others, taking the Chaos Emeralds back into his safe keeping. It was then that he got his first sight of Sonic and had to laugh at his stunned expression as he whisked the emeralds away. Knuckles felt a vindictive glee and striking against the one one who dared have such a strong connection to the gems he was destined to protect.
But the spiky blue intruder was persistent, slowly retrieving the gems and even unlocking their full potential. This had only served to make Knuckles more jealous and drove him to try and stop Sonic and every turn and eventually going head to head with him. But Sonic managed to defeat Knuckles and Eggman had taken advantage of his weakness to steal the very gem that was his duty to protect. Fully humbled, knuckles ended up helping Sonic and grudgingly trust him to retrieve the Master Emerald. To his surprise and reluctant gratitude, Sonic returned the Master Emerald to its rightful place.
He expected that to be the last he ever saw of the blue hedgehog, but he came back. Eggman hadn't returned and there was no threat, he just wanted to get to know Knuckles. Knuckles ignored his efforts at first, but Sonic was persistent and kept coming back. Eventually, Knuckles began to open up to him and even ventured off the island. He then left again, at first as part of his duty to the Master Emerald and then more often to visit. He meets teammates and even became friends with those that dwelt on the surface. Soon he was spending just as much time on the surface as he was on his island.
Knuckles would never admit it, even to himself, but he owed a lot to Sonic. Because of Sonic, Knuckles had friends and a much fuller and happier life then he was ever expecting to have. Then came the day where everything change. Sonic became bound to the Chaos Emeralds and left their world for a greater destiny. Knuckles' duty became even more important as the Master Emerald now served as Sonic's anchor to their world and the only way stay connected with him. Knuckles became even more vigilant, determined to keep the Master Emerald safe for his fiend. It was no longer just his duty, it was his honor. One he would serve to his last breath. For Knuckles was not just a guardian, he was a friend and he refused to let his friends down.