User blog comment:Vivarn/Is Millianna a lesbian, too?/@comment-24066070-20140120010132

She probably fell head over heels for the "must kill Jellal" thing.

IF she is lesbian, she would fall for Erza. Or a cat... or Erza in a cat costume (has been shown several times already)

Another possibility is that she (and perhaps Joe and Wally as well) feel guilty about not being able to prevent Simon's death. Therefore, she wants to "be there" for Simon's sister.