Here you can see a list of character objectives that include obtaining different characters
Dynamic Duo[]
Dynamic Duo is an objective to purchase any two characters from the Store. This does not include Guy Dangerous.
Fantastic Four[]
To get this objective, one must unlock any four characters by purchasing them from the store, excluding Guy Dangerous.
Sexy Six[]
To get this objective, one must unlock all of the characters by purchasing them from the store, excluding Guy Dangerous