We're going to do all three of these kids at once, because, honestly, there's not much difference between them. They're all attractive, kind, educated, and gentle—except when it comes to dealing with the monster.
To be honest, Shelley lays it on really thick here. Take the names. Felix is Latin for "happiness"; agatha is Greek for "good"; and safie is—well, "Safi" is a male name in Arabic meaning "pure," but Shelley was probably drawing on its similarity to the Greek sophia, which means "Wisdom." So: happiness, goodness, and wisdom/ purity. That's quite a collection of virtues.
After meeting this angelic crew, we're not surprised to find out that they have "virtue and good feelings" and "gentle manners and amiable qualities" (13.19). Unfortunately, we don't find out much more about them: they represent (1) all that's good about humanity, and (2) how even the best people are a little bit racist.
Except … notice that Felix's story becomes just one more story-within a story (the monster's)-within a story (Victor's)-within a story (Walton's). When Safie ends up with Felix instead of back in Islamic Turkey, she chooses love over societal boundaries. And when you think about it, that's exactly what the monster wants all of us to do. Is there hope after all?