Of the five Voltron pilots, arguably the most mysterious one is Keith. As was not part of the academy in any form when we meet him. But find out that he was a part of it once upon a time. He was kicked out for "attitude problems" that we never got full answers to in Season 01.
However, it was clear early on that not only was Keith a gifted pilot, but he had a connection with the lions. After his expulsion from the academy, he stayed in the area as he felt a deep connection to some mystical energy he couldn't explain. We would go on to find out that this was the Blue Lion he was sensing. And he would use his knowledge of the terrain to help the others find it out once they met up.
Unlike many of the pilots, Keith had to truly prove himself to the Red Lion in order to be dubbed worthy. To do so, he risked his life and almost got sucked out into deep space.
Keith is a warrior, and arguably is the best one on the team save for Shiro. He often has an aggresive attitude, and takes a while to warm up to people, including the other members of Team Voltron. In fact, it's clear that his time away from the academy helped cement his "attitude problem" as he's often portrayed as the coldest member of the team. Going so far as to do and say things that the others wouldn't dream of saying or doing.
Despite this, the team does count on Keith's skill as a pilot and warrior to help them whenever they need it. It was Keith who tracked down an alient who stole the Blue Lion after he fled into an asteroid field, and helped capture Commander Sendak after he almost stole the Castle of Lions. Regardless of attitude, Keith is a worthy addition to Team Voltron.