On paper, the K2 sounds like a thrilling show with romance and intrigue mixed in - you've got a highly skilled mercenary; a girl whose been kept hidden her entire life; powerful people who will go any length to maintain the secret and a hunt to unravel the past. And while the show does manage to succeed on some fronts, overall it falls a bit flat.
The main problem seems to fall on the directing which focuses more on the flashy and less on allowing character development. The fight scenes are amazing but had a tendency to overshadow and once things calmed down, everything felt flat with little explanation for character motivation (such as why Anna loves hr father so much despite the show never actually a decent interaction between them). The stakes seemed high but never really felt it - we're told Anna needs to discover what happened to her mother and escape the clutches of Choe Yu Jin but everything seems to happen slowly and with a lot of talking. A balance between action and exposition was never reached.
Part of the flaw does fall on Yun A, whose character is central to the plot but whose acting never fully manages to bring her character to life - it's hard to root for her to win and while we're told why she's important, again it never feels like she really is. There's more chemistry between Je Ha and Yu Jin then there is between him and An Na with poor development of the romance portion of the story. So when they do get together it's hard to believe it was natural and didn't occur because we were about half way to through the run time.
Despite the flashy action scenes, the show falls flat on a variety of fronts. A recommend for lazy days but otherwise a skip.