Kaldur was created for the YJ cartoon and was introduced into the comics around that time. Judging from the submission date, this was drawn before the show came out, so it probably predates him. And if it doesn't, I'm betting he'd only just been introduced. Too late to add him into the picture.
He was there before Young Justice, but he was introduced in Brightest Day during June 2010. He was created for the show, but then Geoff Johns took a liking to the character and introduced him into comics right as the show came out.
Third sentence of the first paragraph here. [link]
DC's created characters for their shows before, and a few of them made it into the comics- The most famous DCAU one was Harley Quinn, who was created by Paul Dini for Batman the Animated Series and became a regular member of the Gotham Rogues' Gallery. The most famous case overall though was Barbara Gordon- it's a really close match to Kaldur's situation, too. There had been a Batgirl before (technically, a Bat-Girl), but she had been phased out of the comics and tends to exist in a sort of limbo nowadays- she still exists, but continuity usually says Barbara was the first Batgirl. The character was designed for the 60s TV show starring Adam West, and while she was introduced in the comics before the show came out, she only existed because the producers wanted an official female presence.
There WAS an Aqualad before Kaldur, but he was Garth, a completely different character entirely who's also shown up on the show. Also it should be noted that Artemis existed before the show, but the show version is really this weird composite of a minor villain named Tigress in the comics and a hero named Arrowette from the original comic series.
Definitely!! From what I remember, they gave him the Super-hero ID Narwhal, and he was eventually killed off... But why not bring him back into the fold with the new 52? Course, even though that was a super popular run of Aquaman, I'm sure DC would rather we just think of the current series...
Among others, of course... I always liked the Amazon, Artemis that was Wonder Woman's replacement for a short time in the 90's... Now she's disappeared... Possibly for good... *sigh* And I was also a fan of the Linda Danvers Supergirl (the one that Peter David wrote stories for/altered to make even cooler than her Matrix days)... But we all know she "never existed" now... Same with Triumph... I imagine all of these characters are still in the DCNU, but as civilians and not heroes...