Miranda enters the story during the third disc and replaces Shana in the party. Miranda neatly relieves Shana of her playable character duties allowing her to become prime plot device. At first Miranda doesn't really seem to have a purpose other than being mean.
This does not make Miranda a useless character. While she does not have as much time to develop, it does become clear that Miranda is a sensitive character who has been hiding away inside a shell trying to protect herself from getting hurt by loved ones. At the same time, Miranda joins a party that has been with Shana for a long time. Dart is likely not thrilled that Miranda is replacing Shana and Rose is puzzled why a Spirit is leaving a living host. Miranda also does not make the most graceful first impression. It's likely none of the party members are really happy to have her.
This likely made it harder for Miranda to integrate herself into the group than any other member. Each member joined for their own reasons or because of the death of a party member. Miranda just up and replaced the kindest member of the group for reasons no one can explain. Miranda never really joins the group. Even in the ending, she's apart from the main group. Miranda is never seen with the others during the ending. Both times she is obviously shown, Miranda is giving a solo shot. Miranda knows she cannot replace Shana and isn't trying to. She comes into the party with her own unique personality and has no intention of changing it for them.
While it doesn't seem like Miranda's character is necessary in the game, in the end she was a much better choice for the White-Silver Dragoon than Shana. Shana does fit the healer stereotype much better than Miranda however Miranda makes a far more interesting healer. She's not perfect and she isn't pure of heart. Miranda is an angry woman and this adds a unique dimension to the healer character.
