the seventhIn the wider universe of Kingdom Hearts, Kairi is of great, significant importance. For one thing, she is the seventh and final Princess of Heart: for another, she is the seventh and final Guardian of Light. Both these titles come from an (assumed) universe fairytale story that was, in fact, true. In the original Keyblade war, the first X-blade was splintered into seven lights and thirteen darknesses. These seven lights eventually became the Princesses of Heart. Presently, aside from Kairi, these Princesses are Alice, Snow White, Jasmine, Belle, Cinderella, and Aurora. The Princesses have no darkness in their hearts so cannot be swayed by it, and, likewise, they cannot become Heartless as there is no darkness to convert. Their collective power is being able to open the door to all worlds; the final keyhole. Alike her fellow Princesses, Kairi can also sense darkness, and keep the darkness at bay, though this is largely subjective and dependant on how many Princesses there are in total. It also explains why she looks sick as Destiny Islands is destroyed; there is only so much she could do subconsciously on her own.
Notably, the Princesses of Heart are not described as physical fighters. They are pacifists. The seven Guardians of Light exist for such a thing, to protect the light, and to stop it from being snuffed out by the opposing thirteen darknesses. Part of Xehanort's scheme was to amass thirteen beings of darkness in order to destroy the world; he sets this stage by creating the Organization, amassing a group of thirteen nobodies under the falsehood that they don't have hearts and that this way, they will regain them, and waits for it to fall into place. When it becomes apparent not all of them are suitable candidates for his plan, many are removed and Xehanort then replaces them with others, or further versions of himself. However, he makes a mistake - he tries to acquire Sora and reveals his hand in advance. The side of Light is blessed with an opportunity of knowing Xehanort's plan ahead of time and that they do need to amass their forces, the seven Guardians to protect the Princesses, and end this war once and for all.
These numbers are easy to figure out. Sora, Riku, and Mickey are natural choices, making three. The lost Keyblade wielders of ten years before, Aqua, in the Realm of Darkness, and Ventus, asleep in Castle Oblivion. That's five. And Terra, who has been captured by the thirteen darknesses… perhaps him, as well. Five and one to save before the final showdown - hopefully. But what of the seventh? At first, it seems that Lea may take this role, but as the secret ending reveals in Dream Drop Distance, Yen Sid calls upon Kairi, learning that one of the Princesses can too wield a Keyblade. Kairi is given the option by Riku, who was sent for her, and she returns with him to the Mysterious Tower, willing and ready to stand up to protect those she cares about, from how determined she looks. She's waited long enough on the sidelines. Perhaps one of the Princesses can protect themselves, after all. From here, her training to be a fighter and a Guardian begins.
As I've mentioned, Kairi is indeed a Keyblade wielder, though she has had little experience with it. Kairi spent much of the first few games waiting on the islands at Sora's bequest, in order for her to stay out of danger; yet fate has a way of working against the most noble of ideas - in the end, it was apparent Kairi needed to be able to protect herself, inevitably tied up in the events to follow. Riku first granted her a Keyblade - Destiny's Embrace - whilst she was unarmed in The World That Never Was against the Heartless. Keyblades are only useable by those who have inherited them; in Kairi's case, she accidently inherited hers from Aqua. Whilst she was fleeing the Unversed as a child, she touched the hilt of Aqua's Keyblade, which is essentially the Inheritance Ceremony, though this goes unnoticed by the two of them. What's also important to note is the Keyblade itself plays a part in this; it accepted Kairi as its heir. In some way or another, Riku learns of it, and brings her the Keyblade, to protect herself with. Out of the trio, Kairi is actually the first to inherit her Keyblade, though the last to receive it - it is perhaps possible that she could have summoned it if the need had arose, but, being unaware of it, she never would have sought it. Either way, she is unpractised, her skills not yet honed nor discovered - her fighting style was more so defensive and a mixture of partaking and being a spectator in the sword fights on Destiny Islands. Despite this, against her first group of Heartless, Kairi is able to hold her own and defeat several in a single hit - clear competence - and then work together with Riku to take out the remaining group. It's not something to be sniffed at! It is certain that she will improve and be trained, either by Mickey, Riku, or Yen Sid, instead of being wholly self-taught. Her magic capabilities at this time are unknown, but it's likely she will be a good mage and will probably have access to light spells, similar to Holy (which Minnie uses, also). She is one of the Princesses of Heart, after all.