Board Thread:News and Announcements/@comment-69.171.163.204-20140211230902/@comment-3146930-20140212000921

First off, I think you phrased your question wrong. I think you mean "Why isn't Ruby's last name Long like Yang's?" By saying Ruby's last name can't be Long, it makes it sound like Ruby had no choice in the matter.

Anyway to the actual question, there are a few different ideas about what the answer is. The most common are:
 * 1) Ruby and Yang's father has the last name Long, but when Summer Rose and their father got together, she didn't change her name. Ruby kept her mom's name, but Yang decided on her father's name. It is at least hinted at that Yang is much like her father, as Ruby compares the two in one of the earlier episodes.
 * 2) Long is a fake name. In the trailer, Yang mentions having several names when talking to Junior in the club. Long is just the alias that she goes by the most. As shown with Jaune, apparently Beacon doesn't really pay that much attention to their transcripts anyway so she could have used this name.
 * 3) Ruby and Yang are only half siblings, either having a different mom or dad. One of Yang's parents however would have the last name Long.