Kerilian’s background is extremely nebulous. At most, we know she saw something in Athel Loren while observing a random Spellsinger that compelled her to venture out into the Empire on a personal mission. I believe, however, that Kerillian is not a true Asrai from Athel Loren but rather an Asur Spy from Ulthuan.
First, her attitude. While Wood Elf behaviours are as varied as nature itself, Kerilian’s particular brand of outspoken haughtiness and biting sarcasm seems more in line with what one might expect in an environment of stick-in-the-butt nobles and court intrigue. She lacks the severity one would expect from an Asrai, particular from a Waystalker or Handmaiden. Also, as evidenced by the recently added voice lines, she is extremely intelligent and observant, even by Elf standards. She acts pretty much exactly like you’d imagine a campy over-the-top James Bond style character would.
Second, her acting as a Shade. Shade is specifically a Druchii (Dark Elf) class. Dark Elves are basically Elf Nazis, only more evil and sadistic. (Seriously… f$#k those guys… Sincerely, everyone ever). The Druchii hate the Asur and the Asur and Asrai both certainly despise the Druchii in return. Kerillian, if she were Asrai, would make about as much sense being a Shade as Kruber having a Gutter Runner class or Sienna being able to use Waaagh! magic, or Saltz committing heresy. Know how it does make sense? If she’s actually a spy who’s been trained to blend into any Elf culture/society, or if she actually spent time as an operative in Naggaroth.
Third, Asrai are isolationist in the extreme, believing Chaos will consume the world and that Athel Loren will remain untouched by virtue of its unique and diverse magical properties. They don’t get out much in the best of times. A true Asrai, particularly not a Waywatcher, wouldn’t just up and peace out for pretty much any reason, they’d hunker down and double down on being nebulous shut-ins. While it’s possible there’s another more fantastical explanation, the most logical explanation is that w/e she saw altered the parameters of her covert mission in such a way that she was forced to venture into the realm of man to either gather more information for Ulthuan or to ensure its safety. A wood elf east of the Grey Mountains is pretty much unheard of. Maybe she saw some Chaos-y bs but Asrai, and Athel Loren in general, are so extremely resistant to corruption that that seems unlikely or at least wouldn’t fit the lore.
Lastly, spying is common practice for the Asur. They have spies in the Empire and would definitely have spies in Athel Loren and Naggaroth.
I really like this interpretation and feel it has way more depth than “Wood Elf sees janky magic bs and goes on a quest.” I hope my analysis proves accurate down the road.