WARNING: the following contains spoilers for Sword Art Online Progressive: Aria of a Starless Night, now playing in theaters.
Sword Art Online has a habit of introducing new girls for fans to go crazy over in each new installment of the franchise. The anime has obvious ones like Sinon and Alice. As for the 2017 movie, Ordinal Scale, there's Yuna. Even the video games feature new girls like Strea and Philia. Sword Art Online Progressive: Aria of a Starless Night continues this tradition with Mito.
Mito is in the movie to give it a unique flavor. She's part of what keeps Aria of a Starless Night from being a simple retelling of the "Aincrad Arc." She can be cool, good-humored, and even charming. She also plays an integral role as Asuna's friend and mentor. Her fanbase, however, will be best determined by examining her choices actions throughout the movie.
In SAO: Progressive, Mito, or rather Misumi Tozawa, is one of Asuna's closest friends in real life. It's kind of strange because, on the outside, they seem like polar opposites. Misumi is a pro-level gamer who can attract crowds of people when she plays at the local arcade. Asuna doesn't really play games because she spends too much of her time studying. Ironically, Misumi still ranks higher in overall test scores than Asuna. In their free time, Misumi likes to kick Asuna's butt in fighting games. On paper, none of this sounds like the basis for a good friendship.
As it turns out, Asuna is just as important to Misumi as Misumi is to Asuna. She has trouble getting along with other girls because she's a gamer. Furthermore, her status as one of the top students at her school makes even her classmates too nervous to talk to her. Besides this, she generally comes off as distant and aloof. Asuna is the only one who goes out of her way to spend time with her, so she cherishes their friendship greatly.
The time these two spend together seems like genuine fun. They play games together, laugh at each other's jokes and braid each other's hair. The other girls can only wish they had a connection like this one.
Misumi is also made the one who gets Asuna to sign into SAO for the first time. The original anime simply had Asuna sign into the game on a whim. In Aria of a Starless Night, Misumi invites her to spend time with her in the game sometime. Getting introduced to a game by a friend is a much more believable and meaningful reason to sign up.
When she gets there, she finds Misumi (username: Mito) under hijinx-oriented circumstances. As a casual gamer, Asuna expects her friend to resemble herself in the game world. After all, she herself went with her own name and face. When Mito reveals herself to Asuna, her avatar, username and even her voice are all vastly different from her real-world self. According to her, they're indicative of the kind of character she likes playing as. As far as it is from her actual appearance, it's both cool and a little funny to get this kind of insight into her personality.
From there, the two of them have a good time exploring the Town of Beginnings. Mito is excited by the prospect of showing Asuna all of the best parts of SAO. By Asuna's count, there are a lot of them. Once the girls find out that SAO is a death game, Mito's quick thinking and expertise become essential to their survival. Like Kirito before her, she takes the initiative and leads Asuna to the next town so they can capitalize on its resources. Whether or not Mito should be held accountable for getting Asuna trapped in SAO with her, she certainly takes responsibility for it.
Mito's moral support is also integral to Asuna's survival. When they are attacked by Dire Wolves and Asuna almost dies, Mito is able to save her life but the experience is still traumatic. She snaps at Mito because she's sad and confused by her new reality. Misumi comforts her and promises to teach her everything she knows about Sword Art Online. It's thanks to Mito that Asuna is able to enjoy SAO like the game that it is.
Even within their precarious situation, the girls learn to enjoy themselves. They fight monsters, search for rare items, and level up with the same enthusiasm as things they do together in the real world. At times like these, it's easy to forget that they're technically fighting to stay alive. Mito acts as Asuna's trainer and guides her through SAO, but she's also her friend and that makes her a ray of sunshine in what would otherwise be a bleak scenario.
Unfortunately, something tragic and unexpected happens that threatens to separate the two girls forever. While fighting a large horde of Nepenthes monsters, they get separated and are unable to make it back to one another. Despite how far apart they are, they can still see each other's health bars on their HUDs. Mito sees that both of their health bars are in the red and dropping. Unable to bear seeing Asuna die and realizing she won't make it back to her in time, she dissolves the party and runs away. As regretful as she is, this decision could very well be what splits the fanbase's opinion of her.
Mito's abandonment has several unintended consequences. It distracts the already-distressed Asuna while she's fighting a Giant Anthrosaur and nearly costs the girl her life. Even after she survives, the two don't immediately get back together. Mito doesn't message Asuna because she doesn't want to confirm her potential death. Unfortunately, Asuna is in fact alive and very confused by Mito's choice. The combination of Mito abandoning her, almost dying, and hearing murmurs of how no one has even cleared the 1st Floor yet overwhelms the poor girl. Asuna is reminded of the dangers of SAO, falls into a deep depression, and develops trust issues.
The two don't reunite until the raid against the 1st Floor Boss Illfang the Kobold Lord. Asuna even spots Mito at the strategy meeting but doesn't say anything. It's only after she talks with Kirito about the situation that she makes the decision to confront her old friend again. Thanks to Kirito, Asuna is able to make peace with her friend, and they all team up against Illfang.
Mito really manages to pull her weight during the raid. She's responsible for whittling Illfang down to one health bar, defending the other players from Ruin Kobold Sentinels and providing Asuna with a Wind Fleuret rapier. She also holds off Illfang when Kirito needs a moment to recover, a role filled by Agil in the anime. Finally, she assists Asuna and Kirito with the last series of attacks necessary for finishing off the Floor Boss. Mito's invaluable support in this boss battle is a good chance for both Asuna and the fans to forgive her.
Mito and Asuna may be back on good terms, but things aren't necessarily the same between the two of them. Besides the fact that the latter has become a full-fledged gamer, she also finds her own sense of direction in life. She leaves Mito to go after Kirito but does so on good terms. Mito is sad to see her friend go but she can't bring herself to stop her.
Mito is an interesting addition to the Sword Art Online franchise. As far as supporting characters are concerned, she gets a lot done. She serves as Asuna's friend and mentor and seems like a genuinely cool person to be around. Her motivations and feelings are well established in Aria of a Starless Night, which makes her all the more engaging as a character. She might make questionable decisions, but only in no-win situations where the pressure would be too much for anyone to handle. She has great potential to either unite the Sword Art Online fanbase or divide it.