Maybe you can call out her exits rather than ignore her? This is often used, especially if a character is comic relief, to show that the scene is serious. Consider in Avatar The Last Airbender the scenes where Momo the flying Lemur is not among the gang, and the general tone of those scenes (generally, when there's a lot of serious fighting, Momo is no where to be seen, but as soon as the tension winds down, he returns. This is especially notable in the finale when Momo leaves Aang for the dual with Ozai, but returns once the comet passes and the danger is over). Silent characters rely on over emoting in their performance. When Walt Disney was working on Peter Pan, he actually hired an actress to interact with props that were scaled to the Tinkerbell scene (on of our earliest introductions to Tinkerbell, her standing on the mirror and noticing how wide her hips were, was adlibbed by the actress and Walt later included the gag in animation. The scene also helps define her personality as being fiery, very quick to temper, and easily angered by people who are critical of her.). If the character is important, have her emote reactions to what's going on. In the case of Momo, often he was with Aang when Aang was alone, allowing Aang to "talk" to someone and voice his own thoughts and internal conflicts.
In a similar vein (and at time of writing, totally justifiable in watching) Muppets Christmas Carol is surprisingly considered one of the best addaptations of the Charles Dickens work, in part because Gonzo plays "Charles Dickens" in the film, which allows him to give voice to the story's narative voice, making the film a largely straight adapatation. Paired with Rizzo as himself, it allows for them to also work in Muppet humor. Not only does this help with making the liturature more apporachable to kids (especially the fact that Rizzo doesn't believe Gonzo as Dickens is in fact Dickens, thus helping kids grasp that Kermit the Frog isn't really suffering the very serious troubles of Bob Cratchet, but merely pretending to) it also helps the kids get through some very child unfriendly concepts by telling them it'll be okay (Rizzo openly points out that the first line of the narrative as delivered by Gonzo "The Marleys were dead to begin with" was really not the best way to open up a children's Christmas movie prompting a small discussion on the matter of fact that the book needs this fact to be understood in order for the story to happen.) and even at times tell the Children this is the scary part, we'll see you when it's over, to let the kids know that it's okay to be afraid now, and there is a happy ending to all of this (notably during the Ghost of Christmas Yet To Come sequence, which is where the narrator is largely taken over by Skrooge's onesided dialog with the Ghost. As the scene was meant to scare Skrooge's redemption into him, it does have very dark moments). And they return almost as easily as the Redemption act of the story comes into play and tell the kids it's okay from here on out.
Essentially, it's okay to have a minor character out of frame for a large chunk of the story (maybe she realizes before the hero that the situation is grave and goes for the Calvery aka the fairy army or maybe she's there and provides colorful emotive responses that are funny in to the reader and keep them interested in the sequence that is dialog and exposition heavy. Perhaps she is the personification of the mood of the hero and his compainons and her actions "say" what everyone is thinking (she's not afraid to tell you what she thinks, she just can't speak to you... she's bratty when they're under stress and very moody and tempermental when everyone's down, and she's energetic and vibrant when there is cause for celebration. Her ambassatorial role is important, since you hinge the good will of the fairies in your battle, so I would say she's to important to cut, as it seems like that she does have to make a compeling argument to her queen to get them to take arms against the villains. It also helps to show while she can be a very abbrasive person, she still cares for the protaganists and is willing to go to the matt for them.). Maybe include a moment of personal loss for her, that makes her especially bitchy to our heroes... and when they try to cheer her up, it only seems to make things worse and she flies off and leaves the group... when she returns to the story with the army, you can explain that the attempt to cheer her up made her realize that they still cared for her even when she was a pain in the ass to them and that's what got her to defend them and argue to mobilize the army.