The daughter of two former professional fighters, Himatsu was born in Goldenrod, Johto. Intelligent and fairly strong from a young age, Himatsu was a scource of pride for her parents, and respected (though not favored) over her twin brother, Senshi until certain events following Senshi's first evolution. As the more 'normal' of the twins, Himatsu started to recieve even more attention, which dismayed her, as she would have enjoyed nothing better than to be left alone. The Nakaharas expected the best of their children, and rigorous expectations were placed on Himatsu. High-strung and stressed, Himatsu made a deal with her brother. Since the two looked remarkably alike for fraternal twins, whenever she needed some of the pressure off of her back, she would simply switch with Senshi, who would go to whatever party or event or ceremony in her place, while Himatsu took it easy in the back with the rest of her siblings.
In the hope that she would eventually follow in the footsteps of her oldest sister, pro-battling Piyopiyopanchi, Himatsu was sent to the PCA upon graduation from her middle school. However, this disregarded her own plans of going to a performing arts-oriented school. Disliking this descicion, she subtly rebelled by refusing to learn any offensive moves in her first year.
Himatsu, organized the the point of obsession, likes to have as much control of her situation as possible, in terms of schedules and classes. She is often over-exitable and gregariuos, making plans and developing ideas without her teammates. Her confidence is fueled by years of praise, and she often has to be reminded that she isn't always right (not that she listens). Despite her planning ability, she often runs into social situations without thinking, and while she often regrets it later, she never seems to learn from these expriences.

Senshi is a notorius slacker, and known for being alternately being lazy and hyperactive. His twin sister Himatsu was praised over him, and was better at virtually everything than he way. However, that didn't bother him until his evolution into Marill.
Senshi appeared to be a girl until his evolution, upon which he was diagnosed with a form of 5-ARD common in the Azurill line. However, he is (and always was) genetically male, and identifies as such. This did not stop his parents from being somewhat unnerved, seeing their (now) little boy still liked dresses and dollies and makeup. While this eventually stopped, his parents did have a tendancy to shelf him away in favor of their less troublesome child, Himatsu.
Senshi eventually faded into the background (which wasn't hard, between the eight other children). Himatsu's deal did do wonders for him, though, as he functioned better in highly social situations than she did. He thrived on the attention, and not even prancing around in frilly girl's clothes was enough to spoil it for him.
Despite not being as academically or physically gifted as his sister, Senshi was by her side entering the PCA. Unlike Himatsu, Senshi was highly enthusiastic about this and actually impressed his parents by actually showing some promise in battle, and continues to slowly improve. These developments are starting to upset Himatsu. While she wants her brother to do well, impersonating Senshi was her ticket out of the limelight. Senshi himself is a bit torn between a desire for recognition and the security and predictability of being praised for being his sister, nevermind protecting her from the stress of her expectations.
Senshi himself is as high-spirited as his sister, and though he is calm and rational, he often choses to ignore logic, preferring action. He is very social, and loves being around others. Though he has no real drive to join any other clubs or extracurriculars, Uta Dan has become his main focus, and though he is an alternate, he is kept quite busy and would have it no other way.

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Fun Fact: Azurill is also the first and currently only Pokémon whose gender ratio (75% ♀ / 25% ♂) is different than that of its evolved forms (50% ♀ / 50% ♂). This causes 1 in 3 female Azurill to evolve into male Marill. It is unknown if this is a programming error, or a reference to real-world animals that can in fact change gender.

As you can see by the baskets, this picture was a bit late. XP Still, to everyone out there, happy Monotreme Rabbit/Zombie Jesus Day!!!