Leonard is a member of the Valdelamar family, who have historically been a recognised magical lineage in Spain. During the Spanish Inquisition, however, they fled to hide in England after a horrifying incident involving a family squib. They went back after the threat wasn't as imminent, but ever since they've also had a small presence in the magical community that welcomed them. Enough of a presence in fact that when ten year old Leonardo Valdelamar started being bothersome and pulling just enough harmless revenge pranks on his siblings that his parents got fed up, they sent him to live in London with his aunt and her friend-who-lives-in-her-house. Before this, he'd lived all his life in Spain, in his family's home. It's a lovely estate near a mountain forest where he spent his days playing with his siblings and finding frogs and beetles and plants to look at and learn about, and here were the first instances of his magic. Once, he had a terrible fall that sent him rolling, and his siblings told him the trees had moved out of the way for him, until they managed to catch him. His parents were less impressed than he'd hoped, but all his siblings were glad they'd carried him home crying instead of dead.
His aunt and her friend have been his guardians for the last year, and he's flourished under their care. His aunt Mary is a magical artist who specialises in portraits, and her best friend Amelia is an aspiring art restorator. They took him to very fun and interesting places like museums and shows, and his aunt told him all about Hogwarts. Despite all his siblings going to another school closer to their home, it was really not strange when his letter arrived in English rather than Spanish.
He's a quiet kid, always willing to go on an adventure, and ready to cause trouble if it hasn't found him itself. He likes to know secrets and stories, and is very interested in all that can be learned. Not great with people, but not averse to them either, he's ready to make friends and see for himself what the school has to offer. Before Hogwarts he'd been homeschooled with his siblings, and taught all about the history of his home and his country and their legends, and during the year with his aunt he went to muggle school (after a very important talk about the statute of secrecy and all related circumstances). He didn't make many friends, unused to having to meet people, and went through the year focusing mostly on his studies and art and learning all he could about the world.