I always thought Zero Kiryu's role was a fascinating one, but maybe not for the reasons everyone else focuses on. A lot of people see him as the cool, stoic bodyguard for Kaname, and he is that, but his influence on the plot is way more about creating fractures in the established power structure. He's a human who is, for all intents and purposes, as powerful as a Pureblood vampire. That fact alone destabilizes the entire vampire world's hierarchy and their beliefs about human inferiority.
His presence forces Yuki to confront a different kind of loyalty and love, one that's messy and conflicted and grounded in their shared human past. It’s not just a love triangle thing; it's a genuine ideological fork in the road for her. The plot constantly has to shift and react because Zero refuses to stay in his 'assigned' place as either prey or servant. He becomes an independent variable, hunting vampires not out of some grand noble purpose but out of personal, gritty vengeance and survival, which makes his actions feel more immediate and dangerous.
Honestly, the story would have been a much simpler, more straightforward gothic romance without him. He injects this necessary chaos and moral ambiguity that keeps it from being too predictable.