TL;DR:
Massive amounts of fluff/lore, head-canon, and general fanboism ahead. Proceed with caution. Or skip the text and just look at the pretty picture of Imperial awesomeness.
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Intro:
So as a 40k survivour, I'm used to there being massive amounts of fluff and art to roll around in like an itchy pig in a mud hole. Say what you will about GW (and there is certainly a lot to say), when it comes to fluff and art, they not only provide, they provide in a consistently eye-popping/mindblowing way. Epic characters, portrayed by epic writers and artists.
Star Wars EU is much less consistent. For every beautiful vista and character portrait there's about a hundred crappy ones (in fairness to Jaxxon he did get a better picture, but he's still stupid).
Now, you can't fault FFG in the art department concerning X-wing. Beautiful models, beautiful card art, etc. But I do sometimes wish there was more art and fluff about some of my favourite pilots. (not that I'm saying FFG is failing somehow by not providing these - it's not really within the scope of the game itself).
To help my craving, I decided to make this post in honour of some of my favourite Star Wars canon, and a little of the "head-canon" I've developed that fills in some of the grey areas.
So first off, I loved the Imperials as a kid. I wanted to know their stories. I especially loved getting to know the back stories behind the different Imperials who weren't just nazi stand-ins. I was particularly interested in the ones closest to Vader - what stories were behind the relationship between him and Tarkin (who called Vader "Old Friend", and spoke to him in as familiar a tone as Obi-wan speaks of Anakin)? What about the Pilots who served alongside Vader? What had they done to earn his notice? How awesome must they have been to not suffer the death-by-choking we see so many higher ranking officers suffer?
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Canon:
So who are my favourites which appear in X-wing Miniatures Game?
Obviously, Soontir Fel is the man. I'm not going to insult your intelligence by talking about him like you don't know him. Besides, there's tons of lore about him, all of it awesome. Nothing more needs to be added by silly ol' me. And bonus points to FFG for actually giving us some in-game art of the man himself, not just the ship he flies.
After Soontir comes the (second?) coolest squadron in the Empire: Onyx Squadron. Why are they the coolest?
Because they are versatile: According to the lore, they've won battles in TIE/LN, TIE/IN, AND TIE/sa craft, and they were the first squadron in the Empire to fly the TIE/D.
Because they have names: Three Onyx veterans made it into the game: Colonel Jendon, Captain Yorr, and Rexlar Brath.

Captain Yorr in 3 ABY, just prior to the Battle of Endor.
Because they appear in the OT: That's right, both Jendon and Yorr appear in RotJ, and according to the lore fought in and survived the Battle of Endor. (Yorr is the first person who speaks in RotJ, he's the younger pilot of Vader's shuttle. Sadly, an entire scene of them talking was cut from the final film, but survives in the novelization.
(I suppose you could argue that the 181st fills these criteria too [hence the (second?)], as Fel's Wrath is also briefly seen on camera in RotJ, but I'm sticking with Onyx.)
So now things start to get head-canon-ish.
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Canon + Head Canon:
Lore states that between 1 BBY and 1ABY, the Empire took a serious hit. The X-wing entered service and began eating TIE/LN's for breakfast, and the Death Star was lost with (almost)all hands. It was less like Pearl Harbour, and more like if the Japanese had managed to nuke Washington and New York simultaneously in 1941.

Col. Jendon in RotJ, already a veteran of decades of fighting.
As a result of this, a huge number of academy pilots were rushed into service, including more female cadets as necessity began trumping misogyny. I imagine grizzled veteran Colonel Jendon suddenly being in charge of a bunch of green horn kids straight out of their academy diapers, and whipping them into shape. Among these were Yorr - who in 3 years managed to go from cadet to Captain and gain the coveted #2 spot in Onyx Squadron - and Brath, who I'm assuming wasn't far behind (I head-canon him as Onyx 3, even though lore doesn't specify). I also head-canon two other X-wing Miniatures pilots into Onyx: The Next Generation, namely Whisper and Echo (who I head-canon as women, but PLEASE let's not argue. If you think different, that's cool! Please share below.)
Why are W & E in Onyx? Well...if you were looking for pilots for your apex predator next-gen fighter, wouldn't you go straight to the best Squadron in the Empire, where every pilot had to have survived at least 20 missions to get in and had proven themselves in multiple craft? And who's going to put them through their paces? How about the greatest fighter pilot in the Empire?
Hence the inspiration for this picture. I wanted there to be an actual picture of Whisper, Echo and Rexler Brath, as I imagine them:

(By the way, I only painted the stick figure original concept - not the final product!)
So there you have it. Soontir and Whisper share their initial experiences with the scaled-down Phantom Prototype, as Echo, Rexler and a young Yorr (still only a Lieutenant at this point) listen in. I like to think that on Soontir's recommendation, Whisper and Echo were accepted into the newly formed Sigma Squadron, and fought together in the Dreighton Triangle. Rexler rose to Onyx 1 after Jendon retired post-Endor, and Yorr moved permanently to Shuttle Command before finally being given command of a VT-49.
I did some research and decided to head-canon that "Whisper" took her call sign from the Whisper Birds that populate the forests of her home planet of Null, and that her fierce opposition to rebellion came from her suffering through the Separatist take-over of her home world as a young child. I imagined her to be tall, lithe and elf-like, fitting in with her weak gravity arboreal home planet.
"Echo" always felt like a much more rough and ready sounding character. Someone who had graduated from the school of hard knocks with honours. I like to think she's named after her habit of attempting to sarcastically finish the sentences of commanding officers who she regards as either misogynistic or incompetent, and that her sharp tongue often lands her on punishment detail, but her exceptional piloting skills always ensure she eventually finds her way back into the cockpit.
Anyway, hopefully this will interest some, and maybe spur the community on to more fan art. Feel free to share your own head canon, even if it contradicts mine!
Hopefully I didn't bore anyone to tears.
Stay tuned for part two of my project: Homebrew cards!
- Captain Yorr crew card (so he and Jendon can fly ST321 together with Vader. straight out of RotJ!)
- Soontir Fel: Flight Instructor crew card (for the Phantom)
- Jendon, Yorr, Echo and Whisper TIE/D pilot cards.
- Rexler, Echo and Whisper TIE/IN pilot cards.
Dream list: Jendon & Yorr in Lambda escorted by Rexler, Echo and Whisper in TIE/INs for 100pts or less.
Edit: Should "Fanboy fluff-love" be my title when I hit 1000 posts?