Katerina does not exist on Terry Pratchett's Discworld.She is an incidental character devised for fanfic by author A.A. Pessimal.
In the Pessimal Discworld:-Edit
Katerina is Social Secretary in the Rimwards Howondaland embassy in Ankh-Morpork. In the opinion of her old schoolfriend Johanna Smith-Rhodes, she is a complete girly airhead with precious little to keep her ears apart. Nevertheless, Johanna finds her to be restful and undemanding company to chill out with. Anyone seeing the two together - the beweaponed tough adventuress in mannish veldt uniform, next to the ultra-girly Katerina - might suspect it's one of those pairings you see in the Blue Cat Club on Ladies' Night. (But you do not say this especially loudly where a red-haired Assassin can hear it. Or you're carrying your ears home in a brown paper bag.) They would not be further wrong. As much as Johanna can get exasperated with Katerina's inability to get much beyond flowers and clothes and girly things as topics of conversation, and as much as Katerina is alternately terrified and enthralled at her friend's life as an Assassin, this is a friendship that goes back a long way. They say opposites atttract...
Katerina is seen to marry fellow diplomat Martin Vinhuis, a Second Secretary at the Ankh-Morpork Embassy who is tipped one day to become an Ambassador. It is hinted that Martin and Katerina will follow in the footsteps of Pieter van der Graaf and his wife Friejda - an extremely capable diplomat blessed with a non-too-bright but charming and attentive wife. Indeed, in her day, Friejda too had been a decorative but non-too-bright Social Secretary who made the odd faux pas. Katerina was also Matron of Honour at Johanna's wedding to Ponder Stibbons, alongside Victor Tugelbend as Best Man. The reasoning being that if the wedding iconographs are going to be there forever, you want some good-looking people on them.
Initially appearing in the story Murder Most 'Orrible, she matures somewhat in Hyperemesis Gravidarum, where an unexpected streak of angry badass emerges in her and she gets into a fight, admitting afterwards that she has now seen the world through her friend Johanna's eyes, and it had been fun.