Leon is a very minor character in Tyrant, but he will be increasingly important in subsequent books. He is a Numidian slave. Numidia (modern Algeria and Tunisia) was, in antiquity, the region of Africa west of Carthage, another source of skilled mercenary horsemen throughout the Hellenistic world.
Although Leon is a skilled rider and proves himself as a warrior, he was enslaved by Carthage for his looks, and has served the politically powerful Nicomedes of Olbia as a factor — and a male concubine. When he is manumitted, he has difficulty establishing a place for himself. It’s worth watching the reactions of most of the characters to Leon’s first days as a free man, because they think he is ungrateful to his former master — a master that Leon, if you read carefully, probably feared and perhaps loathed.
Leon’s real name is Amazig, but slave traders didn’t care about a person’s birth name, but gave them a name that the owner would be comfortable with.
For those who care about race, Leon is dark skinned, but more akin to modern Berbers than to sub-Saharan Africans. However, his skin is dark enough to excite comment, especially on the Sea of Grass. Also, unlike the Greeks and like the Scythians, he was born in a horse culture, and thus is a natural rider.