We got a surprise today from GoldenFox51 when, while experimenting with Krystal mods for Serious Sam 3, he found a Krystal-related easter egg in the game! Released in November 22, 2011, I can't believe we didn't hear of this easter egg for seven years!
After going to the Secret Yard of Fame in the first level of the game (Summer in Cairo), the player can look at posters for many (all?) of the game's developers, along with a tagline for each. One person's tagline states:
Anton "Mischievous" Marinov -- Furry for Krystal since 2010
Note: The Krystal model in the screenshot above is a mod. The text and poster behind her are not.
The developer in question is Anton Antonov Marinov, a 3D Environment Artist artist for Ubisoft who has also dabbled in programming and tooling. He has more recently worked on the Assassin's Creed series. His public profile on Artstation actually features Krystal as his avatar! Definitely a Krystal fan!
Don't believe it? Here's a video of kacpi26 playing through the game's various easter eggs, including this one. The poster is shown at timestamp 6:05. (Warning: Language and Blood in other parts of the video.)
V-TAL, who had previously spoken to the the artist, implied that there may be other Krystal easter eggs in other games. I'm going to contact Mr. Marinov about this.
Thanks to GoldenFox51, Jimaki, TheVestige, and V-TAL for screenshots and info.