10. Garth/Aqualad - Aquaman
Now, there have been a couple different Aqualads. Here we're focusing on Garth, an outcast Atlantean with powers similar to Aquaman.
Because of the fact that he was different from others in his civilisation, he was ostracised and grew up on his own. One day, he stumbled across Aquaman, helped him out of a jam, and became his sidekick. Being the Silver Age of comics, he provided a lot of comic relief, often making fish or water related puns and jokes.
As the years went on, Aquaman grew more and more distant and concerned with his own family and duties as King of Atlantis.
Things grew steadily grimmer for Garth - he almost lost an arm, was forced to fight a giant sea monster to the death, had to pick a side in an Atlantean crisis, fought Aquaman, and lost the love of his life Tula during Crisis on Infinite Earths.
This finally caused Garth to snap and he left Atlantis, beginning a rivalry with Aquaman that grew into outright hatred.
Years later, now a superhero in his own right called Tempest, he made amends with Aquaman, but as his equal.