she's physically mature (adult teeth, hormonal stuff, growth plates mostly fused. she might have 1 growth spurt left in her, if she's lucky she'll break 5 foot.) she is simply extremely small in stature and, well, flat as a board. easily mistaken for a child, which she doesn't mind as long as people don't treat her like one.
besides, my character who's her cannon rival/love interest is the biter, Kanika prefers punching things.