"No rest for the wizardly."
Sunburst and Sunset Shimmer tied for first place in my patreon character headcanon poll, so I figured I'd do a headcanon entry on both (although Sunset will likely be a patreon exclusive)~ Rambling ahead!
-Cutie marks are very important in unicorn families, especially in clans that place a strong emphasis on magic itself. Unicorns with sun related cutie marks are Sun-Aligned, and these ponies tend to have magic tied to clarity, knowledge, and truth. As the ultimate sun-aligned pony, Celestia has the power of prophetic visions. Zecora, a sun-aligned zebra, is a seer which a talent for brewing potions that allow one to view the past. Sunset Shimmer is both remarkably intelligent and has an uncanny talent for spotting the weaknesses and strengths of others and exploiting them. Sunburst is a highly skilled magical researcher, with nearly encyclopedic knowledge of most existing spells, magical theories, lore, and artifacts.
-Despite Sunburst's rich understanding of magic, he's a rather poor spell-caster. In some ways he's the opposite of Starlight, who has both a solid understanding of magic and the raw power to brute-force her way through any spell she may be unfamiliar with. Sunburst suffers from a disability often referred to as "glitching", a condition not uncommon in unicorns. The magical pathways connecting his horn to his cerebral cortex are malformed, and the flow of his magic is often blocked and disrupted, causing "glitches" in spells he attempts to cast, and sometimes an inability to perform any spells at all. It can also cause spontaneous, powerful surges of volatile magic (such as the one that earned him his cutie mark), which can have very destructive results.
-Sunburst's magical disability caused him untold stress growing up, especially since his childhood power surge had earned him a place at Celestia's School For Gifted Unicorns, as well as the attention of Princess Celestia herself. Yet it seemed, after the initial surge, that Sunburst was terribly inept at magic. No matter how hard he studied and practiced, he couldn't seem to perform even basic spells. Celestia encouraged him to look to friends and family for support, but sad little Sunburst couldn't bear the thought of returning home a failure, not after his mother had bragged to anypony who'd listen about what a magical prodigy her son was. Sunburst holes himself up in the libraries of Canterlot, isolating himself from everyone. Even the letters between him and his best pal Starlight fade to an end when Sunburst's anxiety grows too great for him to answer her eager questions about how magic school is going.
As soon as he's old enough, Sunburst drops out (before he can flunk out). When news spreads of the return of the Crystal Empire, the young stallion boards the first train available, eager to have a place where he can hide from the world, and his lost potential, in peace. With enough training, he learns to perform decent enough telekinesis, and some of the simpler spells for children, but that's about it, and that is his status when Starlight Glimmer finally finds him, years later.
-Sunburst struggles a lot with social anxiety and compromised self-esteem. It makes it difficult for him to rekindle his friendship with Starlight Glimmer, who has developed into quite the accomplished spell master. He can be shy and avoidant around her, as she reminds him of his perceived failures, even as she reminds him that they could not have restored the Crystal Heart without his help. Sunburst is later appointed the official Royal Wizard of the Crystal Empire, and with lots of talks with Starlight, they eventually rebuild their bond.
They do struggle a bit, however, when Starlight admits to a long-standing childhood crush on Sunburst. Sunburst is flattered, really, but merely views Starlight as a good friend. He awkwardly attempts to let her down gently, but Starlight is persistent, believing that as with everything in life, determination is key. After months of flowery gifts and rather forceful declarations of love and shows of affection, Sunburst finally cracks and suggests that Starlight may perhaps an emotional fixation with him that is just a teeny-tiny bit unhealthy? His exact words are "you're creeping me out a little here, Starlight", which ignites a shade of Starlight's temper that hadn't been seen since the days of her village. An unhealthy fixation? How dare he? This is true love!
Starlight storms out and bumps into a concerned Princess Cadence. After a long, long talk with the Alicorn of Love, Starlight comes to understand that she....maybe, just a little, came to associate Sunburst as a symbol of her long-lost happy childhood, and viewed 'acquiring' him as a way to return to that emotional place of nostalgic comfort and security. She'd cared very little about what Sunburst himself wanted, because Starlight naturally knew what was best. She's used to brute-forcing her way through spells and situations, and had thought the same could apply to a relationship. In doing so, she'd hurt her oldest friend.
At the revelation, Starlight suddenly feels very ashamed of herself.
She apologizes to Sunburst, and vows to get to know him as a pony, not a distant symbol of their idyllic, sanitized childhood. And Starlight expresses for the first time, that she is very, very proud of her dearest friend.
Said friend may have gotten a suspiciously sudden case of the sniffles.
Sunburst accepted her apology, the pair shared an awkward laugh and hug at the absurdity of it all, and then gushed about magic theory over a round of their favorite childhood board game.
-Sunburst is the younger brother of Sunset Shimmer. The two siblings were quite friendly growing up, before Sunset's talent for magic led her to be chosen as Celestia's personal student. After tale after tale of little Sunset's accomplishments in Canterlot, their mother, Stellar Flare, may have let the fantasy of two powerful, accomplished prodigy children cloud her mind a bit, and pushed Sunburst rather hard to succeed in life, to be brilliant like Sunset. This had a damaging effect on meek little Sunburst, but he tries to hold no ill will against his mom (she really does mean well), and certainly not against Sunset herself. Sunset's eventual fall from grace and disappearance through the mirror portal terrified their mother, resulting in her being quite overprotective and clingy with her younger son.
As an adult, Sunburst is thrilled to hear that his sister is alive and well, and he travels to meet her when she finally returns through the mirror portal.
-Sunburst becomes a mentor to young Princess Flurry Heart, and later Princess Skyla. Both sisters are fond of him and view him as a surrogate dad/nanny, as he often was left needing to corral and care for the girls when their parents were hard at work running the Empire.
-Sunburst ends up with Flash Sentry, of all ponies, after a diplomatic mission sees him partnered with Flash as a body guard. Sunburst develops a goofball crush on the guy maybe five seconds after meeting him, and a lot of his time with Flash is spent stammering and dropping things. Flash doesn't mind (finally, somepony that's a bigger klutz than himself!) The two end up becoming fast friends, and later tumble into a relationship thanks to the helpful advice/nosy meddling of Princess Cadence. The happily married pair may or may not have adopted a foal.
Flash's favorite nickname for the great genius royal wizard is "Sunny Buns". Because Flash is a dork.