Name: Yamagishi Youko (山岸陽子)
Age: Estimated to be several hundred years old, around 200~300
Gender: Female
Species: Senri
Ability: The power to manipulate the muscle system. What this basically means is Youko has control over a person or creature's muscles, or non-vitals. She has no power over someone's, say, respiratory or digestory tract, but she can make someone have muscle cramp up with very little issue. She can also make someone's heart stop beating for a few seconds with ease or for one or two minutes by expending a
lot of energy, because the heart is also technically considered a muscle.
Occupation: She doesn't work. Haw haw.
Appearance:

Youko is the one in red, on the left. (For the record, Senri is the one in the middle [that is a guy yes] and Aika is the one to the right)
Personality: Youko is pretty laidback if people could describe her in one word - she doesn't give a poo poo about tact and all that stuff; she will be very blunt if she has to... This can annoy most people at once, and it's usually those who she's close to who have to deal with the fallout. Youko is not beyond utilizing her sorcery to prevent fights from breaking out. Since she's not a fighter at heart, she relies on using trickery and her razor-sharp wit to get her out of situations which are incredibly unfavorable for her. However, at heart, she has always tried to see to it that her children (Senri and Aika) were raised well, and despite her brusque demeanor, she does still have a heart in that corporeal existence of her's. As she's a spirit in actuality, her values are... kind of skewed, as she tends not to think of things in the perspective of those who are mortal. You might even go as far to say that Youko is amoral, but she will simply make the defense that "she lives life how she wants to live it."
Backstory (
hope this answers it, Marokuu / Sonae): It is an enigma to most people how Youko came to properly exist, since people either believe that senri are an aspect of the fox, or rather a transformation of an incredibly old mountain cat. Youko has stated at once that she takes aspects of both, but she disagrees with the label that she should be "virtuous," (since the senri is a fox element that is considered as 'virtuous' as opposed to 'wild,' and Youko is more 'wild,' even her name alludes to this) and instead places her origins as a human who was hexed to become a mountain cat, and through sheer meditation and willpower to become human again, she underwent a transformation to become senri. Through her meditation and following vague Taoist and Buddhist practices, she was able to attain distinct supernatural powers - she discovered that she could control the muscle system of other living beings. In fact, her aspect as a fox element is exactly why she can be somewhat compared to Ran Yakumo in terms of spiritual prowess.
Youko lived her life preying on the spirits of men, using her stunning looks as a human to absorb their life force, and soon this extended to fellow women and lesser youkai, as well, due to her ability to control muscles, she could indefinitely paralyze somebody enough to drain their life force - it was in the nature of the senri to do so. This rowdy behavior roused Reimu Hakurei, who sought it fit to exterminate Youko. And she got really drat close to it too - Reimu was incredibly powerful, and was able to stave off Youko's attempts to defend herself. However, Youko, in her wounded and desperate state from having a shitton of youkai buster needles injected into her, used her remaining energy to literally make Reimu stop breathing for approximately 10 seconds, which was enough for the senri to slink away and escape since she had evaporated into her spiritual form.
Youko, in her wounded state, disguised herself once more and sought refuge in a shop in the Forest of Magic... This was Kourindou, and was disguised as an exhausted woman looking for a cup of tea. It was here that Youko met her husband-to-be, a kindly blacksmith who also happened to stop by hearing rumors that Rinnosuke was selling a "very advanced hammer which would help him perform better." The blacksmith could sense that Youko was not as she seemed, and pressed the senri for information; he learned of her plight, and told her that he would shelter her and try to make sure Reimu and similar figures wouldn't exterminate her. But they didn't know that Yukari Yakumo had placed a gap nearby to eavesdrop on them. It seemed that thanks to Yukari's efforts, Reimu had stopped hounding Youko for her misdeeds. The blacksmith said she was free to use him as life force, but Youko thanked him for saving her life, and said she would settle down instead. Live a new life.
When Youko and the blacksmith were out hunting, they discovered an abandoned child in the Bamboo Forest. They took the child in and she grew to be Aika. Youko decided she wanted a son too, and through ~*~sorcery~*~, had a friend of the blacksmith give birth to Senri. It was through sorcery that Senri looks that girly.
Note: I wouldn't say Youko is undefeatable, not in the slightest. She's very physically weak, and stronger youkai can easily crush her - they just have to survive the fact she can basically cripple their muscles and stop them from breathing temporarily. In a one on one fight, she cannot really lose that easily, but if she has more than one person trying to defeat her, it's very likely that Youko will lose. She's crafty, but she can't account for a flash mob trying to snatch her head, since she doesn't have the mobility that, say, Aya or Yukari do.
Name: Yamagishi Aika (山岸愛香)
Age: 23
Gender: Female
Species: Human
Ability: N/A
Occupation: Hunter
Appearance:

Aika is the girl to the right.
Personality: Aika is incredibly bland, incredibly boring, incredibly one-track mind. Youko often chastises her for being so dull and boring. But that's the way she is, she was neglected as a child after all. But aside from that, she is quite the caring family member, and always has the best interests of everyone in mind. She is very grateful towards her parents for taking her in, and repays that by hunting food for not only her and her family, but for the rest of the village.
Backstory: Aika was born to a clan of warriors and warrior-princesses, but she has no idea of her true lineage, since they were all killed. Aika was orphaned as such, and she was found crying in the Bamboo Forest by Youko and the blacksmith husband. The blacksmith could not bear to see a child like that, so he took her in. Youko and her raised her frugally, though at some point Youko decided that she should be taught how to defend herself. The blacksmith taught Aika all he knew about the naginata, and eventually Aika learned to improvise and became incredibly proficient with the naginata. She had studied under Keine for basic education, but she proclaimed herself to be a warrior at heart. She said that she claimed to have the battle lust inside her. Youko didn't take it seriously, but when Senri was born, he could sense it in her... That desire for the hunt.
Aika then used this desire of her's to good use as she worked as a hunter. Actually, there weren't many animals to hunt in Gensokyo. Not at all. But of what little there was, she would take it. And furthermore, she could discern healthy mushrooms for consumption in the Forest of Magic. That was her role, much like Senri's role was to work at the Teahouse.