Shortly after Zero’s arrival into the Cross household, Yuki accidentally witnessed Kaname feeding blood off of Ruka. This changed the closeness they once shared as Yuki is reminded of the difference between humans and vampires. Now keenly aware that her feelings for him were futile, Yuki chose to distance herself from Kaname, admiring him only from afar.
I really liked the start of this series, and I wish they had ended up together instead of Zero entering the picture, but I guess I can always satisfy myself with cosplays like these as well as the many fanfics all over the internet. Thanks to Mica for sending this in!
Source: Yu on World Cosplay