Spitfire is one of my favorite background characters and it's been forever since I last drew her, so here she is in uniform. These are some of my thoughts about her in my AU:
Spitfire grew up with parents who seemed to care more about her academic performance than how she was feeling. Although they weren't specifically neglectful, and there was always food on the table, Spitfire often felt like her own well-being was less important to them than her flight skills or how she was doing in school. Feeling like she was more of a series of achievements than a daughter in her parents' eyes, Spitfire left home as soon as she could with the goal of becoming a Wonderbolt. She moved into an apartment with two of her friends, Soarin' and Fleetfoot, and the three of them worked long hours and trained whenever they could in hopes of making the team. They were eventually able to enroll in the Wonderbolts Academy and, after years of practice, they became full-fledged Wonderbolts. Spitfire still wasn't satisfied, though, continuing to rise through the ranks and become the team captain in record time.
Even though she had achieved every goal she'd pursued — and gotten to see her lifelong friends become Wonderbolts alongside her — Spitfire still didn't feel like it was enough. She still resented her parents for their lack of emotional support, and she was starting to fear that there was something wrong with her — she had it all, so why did she feel so empty? She was starting to lose the fire she had for the job she'd worked so hard to get — that is, until she met Rainbow Dash. The other pegasus' drive and her devotion to becoming the best flier in Equestria rekindled the passion that Spitfire had always had, and she realized that even though she often felt that her past was weighing her down, it didn't have to define her future.
Despite her drill sergeant persona as captain of the Wonderbolts, Spitfire is actually really chill and laid-back when she's just chilling with her friends, outside of work. In terms of family, Spitfire has one daughter, Typhoon, from a short-lived relationship with Soarin'; although the two briefly contemplated trying to make things work between them, they both realized that they were better off as just friends. A few years later, at an after-party following one of the Wonderbolts' shows, Spitfire met Sapphire Shores and hit it off really well. They are currently engaged to be married.
(For the purposes of this AU, I'm retconning "Newbie Dash" out of existence because I hate it. I only mention this because it also means I'm ignoring any characterization Spitfire was given in that episode.)