She's just a very generic overconfident character who doesn't ever grow or learn. She supposedly has some moral values that cause her to fight, but in the stories I've read of her she just feels like a random assassin character. I think the reason why she kills ghosts is because she doesn't want bad people to live forever, which is a fine character motivation, if a little generic.I'm not up on recent lore or stories, but what exactly is boring about Kaya? Especially since you say she is a cool character design?
Kaya has a lot of good cards, and she looks cool. She's just not an interesting person yet. She needs a story that lets us understand who she is beyond: "pay me and I kill ghosts unless you're also a bad person." I think she has a lot of potential to be a good character, it's just that no one at WOTC has actually used her in an interesting way. Right now, she's just like a plumber who fixes Ghosts instead of leaky pipes.