Q:

What does "one" refer to? 

What does the word "one" refer to in this sentence?  Who/what is “the only smart one” in this sentence?  Is it talking about Linda who is the only smart person in America's favorite cartoon family or is talking about the only smart cartoon among other cartoons? 

The only smart one in America's favorite cartoon family, Linda believes that her real family must live somewhere in Centerville, and she has got to find them. 

A:

One refers to a person.  Out of all the people, she is the only smart one.