Lori resembles most depictions of a mermaid in that her upper body looks human while her lower body is that of a fish's tail. She has long brown hair (held back loosely by braids and a headband), light amber colored eyes and a slender frame. Her fishtail is segmented with dark green and turquoise portions; it is oriented with her vertical caudal fin and other fin protrusions along this front seam. On her top half she wears a red and maroon tunic with dual-layered epaulettes, silver arm braces and an additional red armlet on her upper left arm.
Lori was at the Conservatory when Kaldur'ahm arrived with his friends Miss Martian and Superboy. Lori was non-plussed when Miss Martian mentioned she liked mermaids and changed her legs into a tail. But Lori politely pretended to be flattered.
With her fellow students, Lori Lemaris came to the aide of Aqualad, Superboy and Miss Martian when they were ambushed by purists. For the most part, she had to keep the purists away from King Sha'ark, who wanted to eat a few of them.
Ocean-Master's magic weakened her, though she had more strength left in her than La'gaan, Sha'ark and Blubber. She insisted on helping bringing Ocean-Master down, because if only Garth and Tula fought, the purist would still retain some small victory.
Lori was designed for Young Justice by Character Designer Jerome Moore, who was instructed to make her a young girl. He tried to avoid making her look like Disney's Ariel from The Little Mermaid, and also decided not to use the typical mermaid design, where the tailfin is angled like a cetacean and shaped like a chevron. Instead, he based her tail on those of real fish, so that she must swim using a sideways tail motion instead of a dolphin kick.
He also tried to keep nearly everything she wears as recognizably aquatic as possible, for instance by making her hair band with green kelp. However, in the final episode, the color of the hair bands was changed to pink. Her necklace, vambraces, pauldrons, and the tiara atop her head are fashioned from shells and coral.
In the episode "Downtime", a translation spell cast by Lori Lemaris at the beginning of the episode facilitates the use of spoken English. In-universe, all of the characters from the Atlantean segments (except Black Manta, his men, Superman and the Justice LeagueComputer) are speaking the Atlantean language, but the audience hears English because of Lori's translation spell.