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Mitsumi (Japanese: ミツミ) is a main character of Pokémon Diamond and Pearl Adventure! and the assistant of Professor Rowan. Her clothing implies that she is based on Dawn; however, her hairstyle is clearly distinctive.
Little is known about the early details of Mitsumi's life, but when she was young, the lonely and isolated Mitsumi joined Team Galactic, and was raised to become an Ultimate Fighting Machine that would never hesitate to obliterate both Pokémon and Trainers alike. It wasn't until she met Professor Rowan that she started to change. The Professor taught her all about how Pokémon should be companions and should be treated with kindness, not as tools. He then gave her a Chimchar, which helped her to realize the true friendship Pokémon can provide. She subsequently left Team Galactic, battling Jupiter on the way, and became Professor Rowan's assistant as well as a highly talented Pokémon Coordinator.
One day, the Professor sent her out to find Hareta, a boy who had spent years living with wild Pokémon on Route 201, so that the two of them could go on a quest to find the mythical Pokémon Dialga, known as the god of time. As she set off with Hareta, he and his
Piplup were kidnapped by Team Galactic in Jubilife City and taken to the Oreburgh Mine. She managed to tail them and was planning on rescuing Hareta during the night; however, the Gym Leader, Roark, stepped in instead. After being rescued, Hareta subsequently challenged Roark to a battle and earned his Coal Badge.
Arriving in Floaroma Town, Mitsumi and Hareta meet another Pokémon Trainer, Jun, who immediately lunges for Mitsumi and ends up on the receiving end of her fist. Mitsumi then warns Hareta that he shouldn't go wandering off like he did in Jubilife because there's all kinds of freaks around, like this guy. Jun replies that calling him a creep is kinda harsh as he gets to his feet and explains that he is also searching for Dialga on Rowan's behalf. Mitsumi tries to shut him up by showing off Hareta's Coal Badge, only to find out that he also has one. He then challenges Hareta to a battle, but before they're able to go at it, Team Galactic strike at the Valley Windworks. After Team Galactic is dealt with by Hareta and Jun, Jun once again goes his own way.
After a few other adventures, including Hareta getting his second badge, Mitsumi and Hareta arrive the the Contest Hall in Hearthome City, where they once again meet Jun. After competing in a Master Rank Super Contest, Mitsumi is told by Hareta that he came across a guy who claims he was an old friend of hers, despite the fact she doesn't know of any friends in Hearthome. After parting ways with Jun again, Mitsumi and Hareta continue onwards, as Hareta continues earning even more badges. Eventually, the pair arrive in Celestic Town where they come across Team Galactic as well as Mitsumi's "old friend", Cyrus. Mitsumi tells Hareta that he's no friend of hers and that he's Team Galactic's Boss. As he leaves with Hareta giving chase, Mitsumi stays behind to deal with Jupiter and Saturn. Jun and Professor Rowan soon turn up as backup; however, Jupiter and Saturn leave after being notified by Cyrus that their plans are moving to the next phase. Rowan tells Mitsumi and Jun that Team Galactic must be going to the three lakes where the legendary Pokémon live, and that the two of them must go there to stop them. As they leave, Mitsumi wonders if Hareta's still alive.
Some time later, Mitsumi and Jun arrive at Lake Acuity, where Mitsumi battles Jupiter with Jun standing on the sidelines, amazed by exactly how powerful she actually is. Jupiter seems to have lost; however, with the power of Uxie, she comes up with a plan. She attacks Mitsumi's mind by revealing her past as a member of Team Galactic to Jun and uses the distraction to use her Tangrowth to take him hostage. After consulting Cyrus, Jupiter is told to bring Mitsumi with her. Mitsumi initially refuses, but after learning that Cyrus has taken her Eevee hostage, she realizes she must comply with his demands and return to Team Galactic as their Ultimate Fighting Machine.
Some time later, when Hareta and the Gym Leaders storm Team Galactic HQ, she helps to defeat the Gym Leaders one by one before having to face off directly against Hareta. Hareta tries to rationalize why his friend would be working with the bad guys, asking if it was because of the time he peed on her bag while he was half asleep and forgot to tell her. She briefly gets mad at him, before returning to the cold Galactic exterior. As Mitsumi shows that she means business, Hareta realizes that he's really got to go at it full force to have a chance against her. As Hareta's Pokémon dwindle, Cyrus asks him how it is to be beaten down by his Ultimate Fighting Machine. Hareta refuses to accept Mitsumi as a "Machine" saying that "Mitsumi is Mitsumi!", Mitsumi is touched by this; however, before she can do anything, Cyrus reminds her of the Eevee they hold. As the battle continues, Mitsumi turns her Pokémon's attacks directly on Hareta, prompting his Regigigas to go for her. However, before it can strike, Hareta tells it not to hurt her because she's a friend. Cyrus comments on how good it is to have the Mitsumi of old back, the way she was before Rowan's corrupting influence causing her to leave the team.
As Hareta is beaten down once again with no resistance, Mitsumi tells him that this is a serious fight against Team Galactic's Ultimate Fighting Machine, with Hareta replying that machines don't cry, pointing out the tears running down her face. He tells her that the Mitsumi he knows is a kind person, always thinking of Pokémon and that nothing could ever make her into a heartless machine. Finally free of Team Galactic's influence, Mitsumi is able to finish her battle with Hareta with no regrets. After he wins with the "Piplup and Regigigas Hang Ten Surf", she entrusts him the task of defeating Cyrus once and for all. Jupiter prepares the building for self-destruction under Cyrus's orders, and Mitsumi asks her if they plan to safely release the Gym Leaders and her Eevee. Jupiter tells Mitsumi that as they're no longer of any use, they're free to go; however, Mitsumi herself must remain behind and die to protect Team Galactic's secrets. Mitsumi is fine with this, as she has complete faith that Hareta will be able to achieve what is necessary without her. However, thanks to Jun and his Dragonite, Mitsumi and Hareta are both rescued in the nick of time.
Hareta and Jun head to Mt. Coronet ahead of everyone else to stop Team Galactic summoning Dialga while Mitsumi and the Gym Leaders make more thorough preparations as backup. By the time she arrives, Cyrus's plans have gone horribly wrong as he has lost control of Dialga which subsequently began warring with the unexpected Palkia who has also appeared. Mitsumi and the Gym Leaders, working together with Hareta, Jun, Cynthia, Rowan and even Team Galactic, manage to help calm the two Pokémon, sending them back to from whence they came.
Eventually, the Pokémon League Tournament comes around, with Mitsumi's first match being against Champion Cynthia. As the battle heats up, the over excitable Hareta challenges Koya to a battle in the crowd. Taking a particular interest in this, she decides to forfeit. After a double take from Cynthia who asks why she's quitting, she comments that from the looks of things, it may be more interesting as a spectator than a competitor. Before Koya's match with Hareta, Mitsumi goes and eavesdrops on a phone conversation he has. She asks him afterward what that was all about; however, he refuses to tell her, warning her not to interfere with his work, or else, and goes off to compete in his match.
After the match is interrupted by a terrorist attack by Team Galactic, Mitsumi manages to get a Grunt to divulge information about their new leader, Charon, as well as the fact that they've planted a bomb in the stadium. Thanks to Hareta, the bomb is found and everything turns out fine.