In Fire Emblem 12, Katarina joins the 7th Platoon of the Altean Knights along with Kris, Ruke, Rody, Raian, and Cecille. However, she is revealed to actually be an assassin called Eine who was ordered to kill Marth, but before she can do so she was defeated by the 7th Platoon of the Altean Knights.
She also later appears in the game's later chapters. She appears in chapter 3x where she is shown talking to Kleine before and after the battle. She appears in chapter 10x where she reports Legion's defeat and protests against Eremiya's order for her to kill the remaining Legion brothers. She appears at the end of the chapter 13x where she protests against Eremiya's decision to let Kleine die and stays by Kleine's side as she dies. She appears in chapter 16x where she is forced by Eremiya, against her will, to lead a battle against the Altean Knights. During the battle, if the Avatar talks to her three times then she will join the Altean Knights to atone for her past mistakes. If she is recruited, then she will also make a storyline appearance in chapter 20x when the Altean Knights fight Eremiya.
Fire Emblem Awakening
In Fire Emblem Awakening, an Einherjar of Katarina joins after the xenologue The Strongest's Name is completed, however she does not appear in the xenologue except for when she asks to join the Shepherds. As a DLC einherjar she is a Grandmaster. An einherjar of Katarina also appears in a SpotPass team, classed as a Sage.
While Katarina's name is revealed to actually be Eine, she is still referred to as "Katarina" by most characters, excluding Eremiya and the other assassins, Kleine and Legion, who refer to her as "Eine".
Katarina's text boxes throughout the game, even after her real name is revealed call her Katarina.
In the third support conversation between Katarina and Cecille it is revealed that Katarina has secret romantic feelings for someone.
In the third support conversation between Katarina and the male Avatar it is revealed that Katarina has romantic feelings for the male Avatar.
The fact that Katarina's DLC einherjar in Fire Emblem Awakening has the Grandmaster class is a reference to Katarina's dream to become a tactician.
↑Katarina: I like you, Chris. E-er... I don't mind not being your first. Or even if you like other girls, Chris... If you talk to me every now and then... Just by doing that, you'll make me happy.