Well, the expanded universe is pretty cool, if you ask me. Basically Zekk was a good childhood friend of Jacen and Jaina Solo--the twin children of Han and Leia. He and Jaina had a sort of childhood puppy-love thing going on before he got kidnapped by the Shadow Academy and trained to be a Sith. He redeems himself, practically saving Luke's entire Jedi Academy, and destroying the Shadow Academy in the process. Jaina is of course thrilled, until she discovers that Zekk has no intention of coming over to the light side. Instead, he chooses a middle path, using his emotions to strengthen his force abilities, but not allowing them to controll him. Funny thing is, eventually Luke decides that's the right way to go. Although he advises less use of Force Lightening than Zekk is partial to.
*shrugs* That's it in a nutshell. I was really into the Young Jedi Knight series when I was a kid, but I never finished them. Not really sure what happened to Zekk, other than he and Han went bounty hunting together after... well... that's definately spoiler material, so I won't tell you.