Aqualad/Kaldur'ahm was created for this series. Geoff Johns and Ivan Reis liked the character so much that they subsequently adapted him for the DC comic books (with dreadlocked hair and the alter ego Jackson Hyde) debuting in Brightest Day #4.
Fairly early on in development the decision was made that this was going to be a young DC universe, where the heroes haven't been around that long. Once that decision was made, producer Greg Weisman decided to use Dick Grayson and Wally West as Robin and Kid Flash because he felt that rather than be a strict adaptation of the comic Young Justice, which used Tim Drake and Bart Allen, they were really working with an entire history and legacy which were critical to the story.
Lead character designer Phil Bourassa and producer Brandon Vietti made a conscious decision to try to handle the Justice League with a little more of a classic design sensibility than the Young Justice team. They did this to try to visually show the difference between generations since the series is all about that dynamic.