Sunburst is a male unicorn pony, Starlight Glimmer's childhood friend.
In Butterfly Wings, Starlight Glimmer takes Twilight Sparkle and Spike back in time to her youth, where they witness a young Sunburst and Starlight playing a book-stacking game. When Starlight's move nearly causes the books to topple onto her, Sunburst saves her using telekinesis, earning him his cutie mark. Afterward, Starlight loses touch with Sunburst because he earned his cutie mark before her, and she starts believing that cutie marks ruin friendships.
When Starlight later approaches him in the Crystal Empire to reconnect with him, he mainly acts awkward around her and purports to be an important wizard. However, he later reveals that while he is knowledgeable about magic, he was not very good at it, and he and Starlight promise to stay in touch afterwards.
In the end of the season, Sunburst comes into possession of Star Swirl the Bearded's lost journal, and he plays a role in both releasing the Pillars of Old Equestria from Limbo and saving Equestria from the Pony of Shadows.