After crash-landing in England in 1651, Leandro was stranded on Earth; he claimed that his tribe had been overthrown, his world had been destroyed, and his wife had been killed. He asserted that he was the last of the Leonians, and that he had escaped the fate of his people by fleeing through a portal created by an alien artefact named the Eyes of Hades, unique to his home planet.
Having lost the Eyes of Hades after he fell to Earth, Leandro forged an alliance with the cynical immortal human "Lady Me" to find it. He promised to take the young woman out into the universe in exchange for her help. Eventually, the presence of Leandro's amulet-esque artefact in 1651 attracted the attention of the Twelfth Doctor. This reunited the Time Lord with Lady Me, whom he knew by her true name, "Ashildr."
After the Eyes of Hades had been retrieved from the Fanshawe estate by the Doctor and Ashildr, Leandro revealed himself to the Doctor, who discovered that in order to reach another world through the doorway the amulet would conjure, a death was required.
At Tyburn, a highwayman named Sam Swift was to be executed. Both Ashildr and Leandro were in attendance. Ashildr used the Eyes of Hades to kill Sam Swift and open a portal; this revealed an army of Leonians, alive and ready to invade Earth. His lie discovered, Leandro admitted that he had tricked Ashildr, and was in fact the advance scout for a Leonian invasion.
When Ashildr used a modified Mire repair kit she had in her possession to reverse the death of Sam Swift, the portal she had opened was closed. For failing in his mission to keep the portal open so his people could invade, Leandro was remotely executed by the other Leonians. (TV: The Woman Who Lived)
Leandro had a similar appearance to an Earth lion, although his body was humanoid in form. He wore a crown on his head, and dressed nobly in opulent robes. Though he walked primarily on his hind legs, he could also sprint on all fours much like a terrestrial lion. (TV: The Woman Who Lived)
Leandro's eyes could glow a fierce red, an ability he seemed to be able to control; he used this skill to intimidate local humans on at least one occasion, during his stint assisting Ashildr with her robberies. He also had the ability to breathe fire. As with his eyes, he used this power primarily as a tool of intimidation. When the Eyes of Hades opened a portal into space, Leandro set upon the people of Tyburn, demonstrating his ferocious strength and agility. (TV: The Woman Who Lived)