19 years ago - Kaldur'ahm is born, the son of Black Manta and a Polynesian woman taken as a laborer when Manta claims her island as his headquarters. He is born with hybrid Atlantean biology, effected by the experiments done on his father.
14 years ago - 5-year-old Kaldur'ahm's father Black Manta discovers his existance and takes him under his wing, killing his mother.
12 years ago - 7-year-old Kaldur'ahm helps Arthur Curry & Garth escape Black Manta's forces, destroying their ships. He is taken in by the Idyllis Atlanteans, helping him understand his half-Atlantean biology.
11 years ago - 8-year-old Kaldur'ahm takes up residence in Atlantis.
5 years ago - 14-year-old Kaldur'ahm becomes an apprentice Atlantean sorceress.
3 years ago - When Arthur Curry joins the Watchtower league, Mera becomes an instructor to the Atlantean sorcerers, and mentors 16-year-old Kaldur'ahm, teaching him her watershaping techniques. Garth gives him the name Aqualad, and he begins to defend Atlantis.
1 year ago - 18-year-old Kaldur'ahm battles Nanaue outside of Atlantis, captures him and sends him to Belle Reve. He joins the Titans, and goes with them into space to save Donna.
now - 19-year-old Kaldur'ahm protects the people of San Diego in the chaos after it sinks in Thorne's experiments, accepting the help of Lorena Marquez.
Kaldur'ahm was a completely original character when he appeared on Young Justice. The show was deliberately crafted to be an alternate version of the DC universe, and several of the characters were specifically meant to be alternate versions of themselves. We even met Garth, the traditional Aqualad, when the story went to Atlantis. Kaldur was immediately a fan favorite character, and for good reason. He was twenty five separate kinds of awesome all at once. His character design, abilities, history... everything about him just popped.
Kaldur actually also made his first appearance in the comics at roughly the same time that he appeared in the animated series but with a more complicated origin. He wasn't simple Black Manta's estranged son, but he had spent some time with human parents in New Mexico that had taught him to fear water... It was an origin, for sure, but it was pretty unnecessary. We're simply using the animated series origins.
We haven't really had to alter his story much, other than to make it fit into the Aquaman story that we've created. He's saved from Black Manta by Aquaman and Aqualad before they retake Atlantis, and he becomes a student of their magical academies once the coup is successful. From there, he follows a very similar arc as he does in the series. As Aquaman serves with the Watchtower, he earns the role of Aqualad, protecting Atlantis and soon being invited to join the Teen Titans.
One element that is a very specific innovation of ours is to include the story of Sub Diego (the sinking of an entire city and the mutation of it's inhabitants into Atlantean hybrids), but to make Kaldur the hero. It's a very cool, self-contained arc, it introduces the new Aquagirl Lorena Marquez, and it creates new intrigue in the DC world at large, but by making it center on Kaldur it also becomes this great moment of actualization where we see him become the young, awesome hero he's clearly meant to be.