Katie is one of the first villains to appear in the series and direct rival to Daemon Warrick in his search for Twilight's key fragments. She is nearly always seen with her loyal lackey Tora who refers to Katie as her 'mistress'.
Katie wears dark blue high heel shoes, a dark violet skirt that's angled down over her left thigh, a black belt with red gemstones set across it, a low-cut black sleeveless shirt, a silver necklace with a purple gemstone attached to it, and dark blue bracelets with red gems over her wrists. Being a succubus she has demonic wings and a long pointed tail. Her long black hair rests down past her shoulders while her eyes are blue with green mixed into them.
Living up to the name and reputation of what a succubus is Katie is the embodiment of a power-hungry demon from hell who only thinks of herself. Having great physical strength and vast reserves for her hexing and destructive magic Katie also boasts an extremely high amount of arrogance towards all she comes across, believing herself to be untouchable and undefeatable to any. Whether it be a charming spell used on potential prey, deadly magic to burn or pulverize those she finds annoying, or simply using her brute strength to tease and torture any she wishes, Katie does as she wants freely without remorse or mercy. In her eyes the world is hers for the taking with very few being able to actually stop her, such as Daemon Warrick whom she appears to avoid battle with whenever the two clash due to his strength and wielding a holy blade. Her goal is to collect all the key fragments to Twilight's seal before he does and undo the seal over the hidden city, believing that she'll be bestowed with unimaginable power should she do so.
Being a succubus she is an expert when it comes to the dark magics, with her spells often consisting of powerful blasts of radiant and scorching energy with violet and crimson hues. She's able to drain the lifeforce out of men through having sex, reducing them to shriveled husks afterwards, and can charm women with her mere gaze into submission. Her kisses are able to turn men into mindless slaves just as they can hex women. As a succubus she isn't able to get pregnant, instead she needs to charm a woman and infuse her with the dark magics of hell with a demonic ritual to turn her into another succubus. During her time in the series Katie takes a strong liking to Triska Raylight due to her fierce fighting spirit, always referring to her as her 'kitten' and planning to steal her away to be converted into her slave.
While there's no genuine affection towards Triska, rather she's only viewed as her property and a toy, Katie does appear to show actual care towards Tora who always follows her around. She always speaks to Tora in a playful manner and will not hesitate to kill anyone who harms her. In Act IV when Tora is slain by Jovian & Jacqueline, Katie clearly shows remorse from seeing her death.
An interesting thing to note is Katie's real name. As Tora frequently boasted when introducing her mistress her full name is actually Kathryn Lrya Ellionatia. Not only is this a very distinguished name but as it's revealed through the course of the series nearly all monsters only use a single name, with the concept of having more than one name being confusing and strange to them. Why Katie is an exception to this hasn't been revealed.
Katie is seen from time to time during the first four books while she races Daemon to collect the key fragments in Koskaysil before he does. She and Tora attack the city of Ashwood to steal a fragment from the humans, with her also finding Triska there who she takes a heavy interest in before they flee the city with Daemon and his girls following after them. Later she nearly kills Forrus when she attacked Tora in the woods, something that is also foiled by Daemon appearing and chasing her off. In Act IV she joins the battle in Eston against a deathmare and the gemini, with her arriving just in time to keep the deathmare from killing Daniel, Specca, and Triska. Wishing to keep Triska safe and help Tora avenge her fallen mother Emily who was slain by Jovian & Jacqueline, Katie joins forces with Daemon's group, with herself and Daemon fighting and killing the deathmare while their girls try and fail to slay the gemini. During the conflict Tora is mortally wounded by Charlotte and thrown to the gemini who slay her as well, enraging Katie who tries and fails to kill Charlotte who wards her away from the town with a powerful spell of holy magic.
It isn't until book six that Katie returns and attempts to kidnap Triska from Daniel to convert her into a succubus. Although she does manage to steal away Triska and start the ritual to infuse her with the dark magics of hell she is interrupted when Pip comes to the rescue and fends her off with her magical power. After Clover returns Triska to Daniel's group Katie once again appears before them bearing small injuries from her battle with Pip, claiming that nothing will stop her from getting what she wants in the world and vowing to kill them all after taking Triska back again. It is then that Pip flies down from the sky and rushes towards Katie, stopping point-blank in front of the demon before unleashing a massive torrent of magical energy and completely annihilating her.
Very little has been revealed about Katie's past in the series so far. In Act XI when Kitten is confronted by Kryelle and Arixis it's learned that the two succubi also despised Katie as she had callously refused to kneel to their queen (a beelzebub) alongside them and constantly boasted how she was going to become all-powerful once she collected all the key fragments.
Although it hasn't been mentioned in the series yet her official bio also hints at some past mysteries with Katie. Her home is listed as Rehnder. This was the name of a human village mentioned in Act I among Daniel, Alyssa, and Triska, that being how Rehnder was completely overrun with monsters and burned down in one night with all the men being abducted and the women being killed. When exactly this happened isn't stated, though it does raise the question of when this was Katie's home.