I-19 imo is still a young girl ( i prefer calling her a child, a cute one^^). Just look at her expression and the way they illustrate her, and simply having big boobs dont make one being mature, i assume. Anyway, look at her quotes, she is a very good girl
I hope she's our sister!No! It's not what you think!Yeah... good... I think it looks good!A new girl is coming? I'm so excited!Ah... No... This is...Stop it!!Excuse m...Nagato