...as Neoteny is not allowed in Missions, for my character Leela, I just used distinctive style to describe that she looks younger than her actual age and expresses herself, like a young girl even though she is 17 and has a LOG of 6 as well as INT of 5.
The Group is fine with that.
Think of Leeloo from 5th Element.
...and yeah, "Beeeeg BadaBoooom!!" is one of her lines.