Rukia was in love with Ichigo— it was so simple to see that it broke Renji's heart. She was his first true friend and the first person he ever loved. Their lives coincided with one another, but every time Rukia would smile, Renji felt his heart give out.
She would never accept nor understand his relationship with the teenage boy. Rukia would never smile and make jokes or smack Renji upside his red-haired head.
But, if Renji Abarai had to choose between his friend and lover, he would pick Ichigo Kurosaki and his hair of sun-spun orange every single time.