Shit.   Is the plot contrived to make her remain a permanent loli?   Is it doing that whole thing where she can't fundamentally change her character appeal type because it will change the entire tone of the story?
And here I figured she would just age far more slowly than humans because she is 75% spirit, but the plot actually straight up states that spirits don't age beyond their early teens or whatever.   It would be too cruel if she is permanently stuck as a loli. 
That said, if she gets good enough at her magic, she could probably add to her own bodymass and artificially change her base form.  That obviously won't happen though.