Merida is completely self-serving and irresponsible, while Snow White is maternal and dutiful.

It´s allowed to add another one, but it would be good if you do it,explain why

Merida is somewhat maternal and dutiful as Merida does get maternal towards her younger brothers and tries to fulfill her duties until it comes to marriage. Merida does try to go against her own nature more than she gets any credit for.

Ariel however is actually not maternal at all, she doesn't really look after anyone else in the entire film except A) Flounder after she lies to him to force him to go into a situation that he doesn't want to be in & B) Eric after she falls in love with him. Also Ariel doesn't really care about fulfilling her duties at all since she A) constantly misses singing rehearsals & B) completely misses the show while she is out breaking her father's rules of going to the surface.

I don't think any of them are truly her opposite, but I'd say Mulan or Elsa (much more so the former) because Mulan struggled so much to do what was expected of her- to be graceful- and for Snow it's no effort at all. Mulan is more intellectual and doesn't have many of Snow's social skills. I think they'd get along, though (of all the princesses I think Mulan and Snow both would be the least judgmental of all the others and would be friendly towards them all in fact). with Elsa, she's also ladylike and prim, but more introverted, more fearful, takes action less, is very cautious, etc.