Why is Kristoff considered a Disney prince?

Cause he was made from disney and anna is a princess and she and kristoff are a couple so they count kristoff as a disney prince.

11Catlover11 is really right on this. If Anna joins the Disney Princesses, then since she and Kristoff are a couple, then Kristoff would join the Disney Princes.

Besides, Flynn isn't a prince, but he's in the line up, and same for Shang and John Smith, etc.

Because he fell is the canon love interest of a Disney Princess, and can potentially be a prince if they ever get married.

Same deal with Cinderella and Belle. They married into royalty :) Although, I don't know if Kristoff is considered a prince yet, though, because I don't think he and Anna are married yet

It kind of like Mulan in the Disney Princess line up. She not a princess but the Fans have just accepted them and shown love to them. So then there in the line up from fan demand. :D