I've been thinking Katie would find him on the incredibly magical scale. The interesting thing is that the sidhe are definitely a race in Katie's world, too, though there's no such thing as un sidhe-- just sidhe in both Seelie and Unseelie courts. And chimera are abundant in Katie's world, too-- in fact, the echoes that she sees here are actually chimerical creations. Given enough time and enough creative force/obsession behind them, they become sentient creatures in their own right. (Until then, they have about the permanence and consistency of a soap bubble.)

So I wonder if any of this matches up! Kai clearly isn't a chimera as Katie would consider them (everyone can see him, for a start), but I suspect given his heritage he might register to her as startlingly close to her kind of fae.