Leroy is a Heartless experiment clone of Stitch, and is technically experiment 629 in Leroy and Stitch.
Leroy was the last of Jumba's experiments to be built. While spending a few years on earth he had an unfinished experiment at his laboratory in deep space. After Lilo and Stitch successfully captured all experiments and turned them from bad to good Jumbaa regained access to his laboratory. During this Jumba's evil former partner Hamsterviel has escaped prison and gave Jumba a visit. He ordered Jumba to finish the experiment as a red replica of 626 (Stitch). The orders were fulfilled and Leroy was created and later cloned to create an army.
Leroy disguised himself as Stitch to gain access to the BRB (Big Red Battleship). Once on board he revealed his true form. While Hamsterviel and Gantu took over the Galactic Federation, Leroy journeyed to earth and captured all experiments with the exception of Gantu's lackey 625 (later known as Reuben).
With the experiments captured and Hamsterviel in control of the universe, Leroy waits on Earth as the experiments prepare to be eliminated. Just before they are Sitch and the Ohana arrive. Leroy and his clones battle Stitch and his cousins. They seem indestructible but due a glitch when being created the only thing that can defeat Leroy is hearing the song Aloha O. Stitch and Lilo perform the song and the Leroys shut down. They are then sent to prison alongside Hamsterviel while the reformed Gantu joins Lilo and Stich.
Role in the Series
In the Spongebob and friends adventures, Leroy is revealed to be a Heartless, and when Aloha Oe plays, Leroy is killed, and his heart asceneds to Kingdom Hearts.