Board Thread:Speculation House/@comment-27883360-20160305074527/@comment-27082610-20161109045851

"Semblance is the manifestation of one's innate and personal power as an ability unique to each individual, with the effects varying greatly from user to user. The nature of one's Semblance is noted as representing an aspect of their character."- wiki page

Bascially your Semblance is something that is based upon one of your primary character traits. And if Ren is concealing his true feelings (for no explicable reason), then it's probable that he'll have something illusion-based.