Udoshi, Kimi - The Pyon Pyon Princess!

'Under Pressure' is quite simply the ability to alter the state of any fluid, in this case referring to gas and liquid in Kimi's immediate vicinity by changing it's pressure. This pressure can then be released all at once or slowly over time, similar to a pressure chamber in a gas valve. If released all at once, it can cause an incredibly powerful and dangerous explosion depending on the level of compression and the medium being compressed.

The specific effects of this change depending on the medium it is being used on, of course. Liquid tends to be easier to compress and has more powerful properties but is harder to work with in a practical sense. Air, however, while needing much higher pressures to do anything useful, is much easier to work with.

Kimi's creativity with this power hasn't quite reached it's peak yet, in fact not even close. She has learned to use it to form 'bubbles' of compressed air around her body that can be released to give her physical attacks intense explosive impact or to change direction suddenly in mid-air; she can also do this underwater by compressing water instead of air. When these 'bubbles' are created, they tend to stay only a few inches from Kimi's body, creating or keeping them any further away than that would require a Range ability.

Compressing the air generally creates a 'bubble' of air slightly smaller than a soccer ball at the indicated point; those created from water or other liquids are generally around golf ball sized. Theoretically further compression is possible but this creates a lot of instability and bubbles are likely to rupture and explode prematurely without proper control.

When these air pockets form they are quite visible and produce a low hissing sound but when they impact with an object and the pressure is released, the burst of air produces quite the explosive noise. So loud, in fact, that continued use of this power in this way is likely to damage Kimi's hearing by age 20 and in fact has already started doing so.

The limitations of this Quirk are rather extensive despite a rather broad power set. In particular, she can only use this power on liquids and gases, not any sort of solid, plasma or any sort of energy. She could not, for example, control the pressure of fire but she could control the pressure of the air around it. She also cannot use the power on living things with one exception; herself. Increasing or decreasing the pressure of liquids and gases within her own body is possible but obviously highly dangerous and ill-advised.

1. Impact Driver! - [ Strength/Damage ]
   

   Despite having such a small frame, Kimi is surprisingly strong and utilizing her Quirk can punch with explosive force equivalent to 600kg of force, enough to shatter thin concrete walls. Having a compact but strangely powerful body, she is also capable of lifting similar amounts of weight, or at least juggle them in the air with explosive power for a short time.
   
   

2. Explosive Entrance! - [ Mobility ]
   

   Utilizing controlled pressure explosions in the air or in water, Kimi can move with sudden urgency through these mediums, changing directions near instantaneously, or gaining height in the air. This gives her a crude but effective ascension and movement ability, though descending is still dangerous and requires either precise control or luck. Combined with her natural agility and lithe body, this gives her a lot of mid-air freedom of movement.
   
   

3. Big Pressure, Small Package! - [ Durability ]
   

   For many reasons Kimi is rather a natural at avoiding being hurt or when she is hurt, taking it on the chin and continuing to fight. Her Quirk places a great amount of strain on her body, especially her bones, so she's used to handling pain; on top of the fact that she can use her explosive bubbles to cushion impacts, or even just use her small size to duck and weave around attacks.
   
   

Disregarding the usual effects of stamina loss, Kimi's quirk still has a great many weaknesses to it, minor and major. In fact it is a rather dangerous and unpredictable Quirk, as it can be affected very subtly by changes in atmospheric air pressure, heat, the presence of gases in the air, etc. Without a reliable way to account for this, bubbles can explode prematurely or much more violently than expected, even having additional unintended effects like heat, fire, sometimes even the generation of electricity through the piezoelectric effect.

Even when in ideal conditions, the Quirk is difficult to manage. Often even before Kimi runs out of stamina she is likely to lose consciousness and collapse; her body has little to no protection against her own explosions. The only reason she is able to endure multiple of them already is that her body has naturally acclimated at least a little to the shock. When she hits someone with one of her enhanced punches, the shock travels back through her arm in turn, damaging her bones and joints and causing significant pain that has to be carefully managed.

The long term effects of this aren't known but it's very likely she'll end up with a lower bone density than a regular adult if she over-uses her Quirk. In fact a lot of the drawbacks of her Quirk are long-term damage effects on top of the immediate pain and wear and tear; many parts of her body are affected long term by her explosions. Hearing loss, muscle spasms, deadened nerves, all of these are risks that in some cases have already begun to manifest.

The powers themselves seem to have a weakness against any power that could affect air density. The temperature of air in particular makes it hard to predict the density of the air and thus how high or low she should pressurize it; fire/heat or ice/cold quirks therefore are a significant weakness of Kimi's.

In addition, she has no control over the density of anything solid or made of energy, which means she has to be very creative in her approach against those who use such objects in their attacks.

Perhaps the biggest weakness is also the most simple. Anyone with aerokinesis, or any sort of direct manipulation ability over the medium she's currently affecting has a massive advantage over Kimi, as she has no direct control over the air or water she's using. it will passively stay close to her body and theoretically could be controlled to stay in one place but aerokinesis or hydrokinesis would override that, as they have much more precise control. This is not to say she could do nothing against such targets, explosions can hurt anyone after all, just that she'd have to be more creative in her approach.

Having been to kickboxing lessons as a young girl, Kimi has taken this knowledge and adapted it in an attempt to expand her fighting capabilities, often incorporating her explosive bubbles to lend sudden power to her attacks. She's also not at all afraid to fight dirty if need be.

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Being an idol-in-training, Kimi is a fairly accomplished singer and enjoys the craft despite having an unusual voice. Although she generally doesn't have the attitude of an idol, her singing is often praised as being high quality. There is still definitely a lot of room for improvement however.