The last secret character of the Shooty Skies Christmas Update was a big mystery. Until the 25th of December neither the name, nor the design of this character was known. This is because this character could not yet be unlocked in game. The developers gave a hint on twitter how to unlock it: You have to collect the daily reward every day 12 days long. If you started doing so on the day the update was released (Dec 14th) the Secret Character would have been unlocked on Dec 25th as a christmas present.
Of course Yuley can still be unlocked by collecting the daily reward 12 days in a row.
Unlock Guide for all Secret Characters in popular mobile games