A Yulen believer is a t.A.T.u.
fan who doesn't want to believe that the 2 female members of the group were not romantically involved with each other. Yulen comes from taking the first 3 letters of each singer's name:
YULia and LENa
Though both singers deny this relationship existited there are still many Yulen believers amoung fans around the world. You can find them filling up forums with 'fan-ifctions' that involve the girls being in some sort of loving relationship.