How is Kusaka yandere? Yandere is someone nice, quiet and harmless, which turns out to be capable of going ballistic and dangerous given the right circumstances.
Yuka was Yandere because she was nice and quiet, but when put under the pressure she snaps and murders people.
Kusaka is not, because he is aggressive and dangerous all the time. He just pretends not to be like that. Yandere do not pretend. They are so bipolar they can go from innocent flower to serial killer in a snap and be honest in both forms.