Heterochroma

Heterochroma is a rare condition/genetic mutation Werewolves can be born with, which causes them to be especially agressive,

Symptoms
A telltale sign of a Werewolf having Heterochroma is by them having two different colored eyes. Haru's left eye becomes blue when his Heterochroma activates.

Haru is seen to gain enhanced strength, speed, and ferocity when his Heterochroma is active, this may be in part due to the full moon, as it is said that the eyes of a werewolf with Heterochroma draw power from the full moon.

Cases of Heterochroma
The only 100% confirmed example of Heterochroma is Haru. It is unsure if Hook still has symptoms as his and Haru's mother gave him a scar on one of his eyes.

We also see Haru lose his Heterochroma when he is young due to losing his temper and fierce nature.