What is the inverse of "guardian"?
Solution 1:
Your dependent or ward (credit to amcnabb), both of which are legal entities, but both of which apply outside the legal sphere.
Dependent connotes a relationship in which the guardian is both provider and protector, whereas ward implies that the guardian is the protector, but not much else.
Solution 2:
I like charge:
: a person or thing committed to the care of another
from wiktionary:
The scope of someone's responsibility.
The child was in the nanny's charge.
Solution 3:
A traditional word for this would be "ward" (for example, in Dicken's Bleak House).