Mei is capable in two styles of ancient martial arts, and their various forms and weapons. In addition, she is capable of extensive control over her ki, and this makes her effectively superhuman. However, these abilities all have hard rules, which I will detail here.
Mei is capable of augmenting her body with ki. The presence of ki in a striking limb will increase its power, and by causing it to 'overflow', the reach of the ki can extend a bit past her body, striking the enemy and hurting them even if her limbs do not touch. Thus the range and power of all of her attacks are augmented, which, as she is a small woman, is very useful.
Physically, without the use of ki, she is actually rather thin! But though it doesn't show, she has the strength of one who has been training in martial arts all their life. She can turn off the automatic limiter in the brain that stops one from using their full strength, like most martial artists. However, her small build is one better focused on speed than strength, and rapid attacks are where her real strength lies.
And her strength and speed are used together to give her a very flexible body, which she uses to be very good at evading attacks. Her fighting styles, Baguazhang and Lung Tong, both stress evasion as defense, rather than actually taking a hit. As a result, her evasion is very, very good. But unfortunately, all this training on evasion has left her very bad at taking hits. She is aware, in theory, of how to roll with an attack, and how to move to minimize damage! But she has never practiced this, because pain hurts and she doesn't want to have to explain to her peers why she is all beaten up.
Ki can be used to enhance almost anything. As mentioned above, it can enhance strength and range in a striking limb. When focused to the point of impact, it can help guard against attacks (again, Mei has not really practiced this). It can be used at the joints to increase flexibility, or into muscles to increase speed, or into wounds to accelerate healing. When used at the leg muscles, it can increase one's running speed quite a bit! But nothing too superhuman, as the legs are simply not built that way- The upward motion of pushing against the ground, going too fast, would simply leave one unable to move as they would not be touching the ground.
There are other effects that can be done as long as one is trained properly. Tiger Kung-Fu, for example, manifests claws of ki, sharpening the ki and slashing with it. Dragon kung-fu, which Mei trains in, does something similar.
It may help to think of ki as a mixture of spiritual energy, physical energy, life force, and mental power. Without extensive training (like Mei had) controlling ki at all, much less generating more, will be extremely exhausting. One's ability to use it is like any other ability- Train it and it gets far better. Forcing oneself to over-generate ki past one's limits, however, leads to burn-like wounds all along the body, and inside as well. It is not pleasant! Continued overuse of such tends to lead to accelerated aging. However, on the inverse of this, a true master with control over their ki does not age unless they expend great amounts of ki. It is theorized that one with sufficient ki levels could even reverse their aging to return to their prime, but acquiring that level of ki is generally considered impossible.
Ki is normally invisible. To make it visible requires massive power, and strong emotion. As ki is connected to one's mental state, emotional duress that increases one's fighting spirit can also cause it to be made manifest. One who has the ability to see supernatural things like ghosts, or see people's auras, however, would be able to see it at all times. Things like color and intensity are also dependant on mental and physical state.
However, once one is capable of using ki themself, they may train and become able to sense strong ki in others. This is not a natural consequence of ki use and must be specifically trained for. Some styles (like baguazhang) are naturally more suited to ki sensing, so Mei is quite good at it.
Now for the martial arts. Mei is trained in two styles; Baguazhang (Eight Trigrams Palm) and Lung Tong (Southern Dragon Kung-Fu). In addition, she is ranked in Baguazhang's knife styles (Eighth dan), staff (sixth dan), and spear (second dan). The tenets, much like normal baguazhang, involves walking the circle around your opponents! However, with Mei's form, this circle has a different purpose.
While walking the circle, the baguazhang practitioner (i.e. Mei) projects a diffuse cloud of ki towards her opponent. When the circle is complete, the opponent is surrounded in a cloud of Mei's ki, enabling her to sense everything inside that cloud, down to the smallest movement. In addition, she can sense the movement of the opponent's ki, even to the unconscious use by non-users of ki, and due to ki's nature, she can sense their intentions. In other words, she can generally tell the mood and basic impressions of thoughts from the opponent! This use is the origin of the 'eight trigrams' part of the name, as it is used to track energy, and the ability to respond to moves the opponent has not actually made yet gives the impression that the user is divining the future.
However, this has a few weaknesses! Namely, if the person can stop the circle from being completed, it forces a blind spot in her senses. If the person can escape the circle, she has to expend that much more ki to make another one. The circle technique is also designed for one-on-one battles, so if the opponent has a friend, it's almost useless. In addition to all of this, if the opponent either has physical capabilities she cannot match, or the capability to clear their mind and fight without thought or intent, the technique's use is greatly reduced.
Other than that, her style takes traits from all other styles of baguazhang. However, it can most easily be mistaken for a variant of Fu-style. The spinning motions and precise strikes are emphasized. The spinning is to keep constantly in motion, and deflect and evade all attacks. The precise strikes are to be aimed at pressure points and vital organs. Occasionally ki will be used to add to the strike, attempting to force her own ki into the opponent's ki system, disrupting their ability to move and fight. Otherwise, it can simply be forced in a physical manner, causing serious injury to the vital points. This is not usually done, however, as energy spent striking is energy that cannot be used to defend.
Its weapon styles mostly come from Liang-style baguazhang, and employ its philosophy of form-switching quite well.
It uses the forms of Tian-style as well, for completion. It is a very well-rounded style, though focused more on defense than offense. Its offense is designed to be very fast, and use minimal energy, shutting down the opponent with minimal damage.
Her other form, Southern Dragon, is much more straightforward. It is another evasive style, but this one is much more offensive. It emphasizes full-body technique, moving power from the feet through the waist to flow out through the fist. Speed and power are very important!
In addition to punches, it uses tiger claw techniques, leopard hit-and-run techniques, and low sweeps. However, its kicks are rather limited! All of its arm moves are pinpoint strikes, however.
Unlike baguazhang, Southern Dragon uses as little movement as possible when evading attacks. It uses rotation or torso without moving stances! This is good for saving energy, but bad against wide-reaching attacks. She is very quick, however, and can shift stances in an instant to one better-suited for evasion.
While as with normal Southern Dragon, the forearms are the primary place of defense. However, unlike that one, it is not necessarily the physical forearms that take the blows! Instead, the forearms are covered with a hard armor made of ki. For a novice, these are hard, solid blocks, but for a master, instead they take the form of dragon scales, offering more protection from the overlaying scales while softening the blows more, and requiring less ki to maintain overall. This way, the forearms are still used to block, but they are protected, so any risk to them is severely lessened.
The ki tends to be used much more quickly and offensively in this form, however. It sharpens the fingers for tiger claw techniques, is used for precise joint blows and increases flexibility for joint locks, lengthens the reach of a sweep, and so forth. However, unusually, it also possesses a few techniques, Hidden Secret Arts, of which the ki is the primary attack method. These techniques are considered dangerous and incredibly powerful, able to kill with ease, but also incredibly taxing and draining. As such, they are considered forbidden techniques generally, only to be used as a last-ditch effort, or in special circumstances.
The first, Golden Advent, is simple. It shapes one's ki into a dragon, and the dragon is then controlled through the user's arms. Being a solid mass of ki, it has no definite striking range, able to lengthen and constrict at the user's will. Its head, controlled through the right arm, uses very, very sharp teeth to tear chunks out of its target, and has great physical strength backing it up. In addition, the horns may be used to pin or smash. The tail, controlled through the left arm, has a wide, flat tail, able to function as a shield or as a whip, or with focus, as a sword to slash cleanly.
In addition to all of the above, the dragon's size means it extends from roughly Mei's chin to her bellybutton, jutting out and forming armor to the upper torso. And on top of that, if one lacks the ki to extend it far enough, the dragon can be detached from the user and launched as a missile, with a preset set of instructions. However, this is generally a terrible idea.
Its pure size means its ki requirement is vast, especially if one does a lot of extending and retracting. If the dragon remains attached to the user, after it is done, much of the used ki can be reabsorbed, but any ki spent to extend or retract is gone, as well as some loss and mental fatigue from using the technique in the first place. And of course, if the dragon has been used as a projectile, it's simply gone forever.
The second, Heaven Rain, uses a massive amount of ki fired upwards. It disperses up above before condensing into a cloud, and then it rains down in an uncountable number of tiny raindrops. Which would be rather mundane for such a hyped-up technique, except that these raindrops each have the force of a light blow. When you consider the number of them, that is rather impressive! But still not quite enough to be a forbidden technique, right? Which is why each raindrop also explodes on impact, dealing massive damage. Even the user is not safe from these explosions, except by starting the technique with a hole planned in the middle for them, or by setting up a wall of ki over them. And generally, one who has used this technique does not have enough ki left over to sustain a shield against all of this! So without careful planning and control, this is basically a suicide technique. Mei's maximum range for this technique ('maximum' in this case being the point where it will work, but it will completely incapacitate her if not kill her outright) is a fifty foot radius from her. Maximum range usable in battle practically is closer to twenty feet radius. And doing it in a way that will leave her in fighting condition afterwards shortens it down to about five feet radius.
Her teacher had a third technique, Golden Warrior, which used the Golden Advent as its first step. He would then convert this dragon into full body armor, increasing his offensive and defensive power many, many times over. However, the cost for this was an extremely high Ki output, and the user's speed, agility, and flexibility was severely crippled as well. As such, Mei does not care for it, and once she learned it, has very rarely even practiced it. She learned the technique so she can pass it down! But the benefits just do not outweigh the downsides to her.
So, basically, she has one style that is very good defensively, and one style that is very good offensively. She can switch between them, though it is hard to do so rapidly! She is also capable of using the modern forms of both martial arts when she needs to hide what she can do.