Renji hates Ichigo.
He hates the color of that bastard's hair, he hates the bastard's way of acting without thinking, and he hates how strong the bastard can get after a mere few days of training.
He especially hates it when Ichigo is with Rukia.
Because he hates how similar he is to Ichigo, and he hates how it could be him, Renji, with Rukia, instead of Ichigo.