We never meet Leander, but the twin brother Loyd lost at fifteen is still a major figure in the story. His death is probably part of why Loyd was such a jerk in high school, and certainly, losing Leander is a big part of what makes Loyd the person he is as an adult.
Leander was apparently a pretty wild teenager who started going off the rails after the twins left Santa Rosalia to live with their alcoholic father. He died in a bar fight as a young teen. Bummer.
Even so, despite his sad, short time on earth, the artifacts we see of Leander's life aren't about recklessness or violence; they're about his intense connection with Loyd. We see a photo of the two of them, identical on horseback, and we see their two matching handprints on the walls of a cave.
It turns out that Loyd losing Leander is a lot like Codi losing Hallie. Leander's absence turns Loyd into a person in grief for his other half. That's part of what makes Loyd the only person who can really understand what Codi is going through.