Why can Kairi use a keyblade?

I Think Cause She's The Princess Of Heart And A Pure Hearted Girl And Riku Gave That Keyblade To Fight For Herself When Sora Or Riku Isn't There When She's In Danger

It's actually unknown if Kairi can keep the keyblade she receives from Riku, the game only shows that she has the ability to wield one.
It may have to do with the fact that she's a Princess of Heart (her heart possesses no darkness, she has the powers to hold back darkness), or that she has connections to Xion, who also has the ability to wield the Keyblade.

I think it's because of what happened in Radiant Garden where she met Aqua and Micky. That OR it's because her heart had been mixed with Sora's AND Ventus's hearts, exposing her to enough power to wield a Keyblade but not summon one

Kairi can use the keyblade because she is the princess of heart, she has a pure heart and is bonded with soras heart, therefore she can wield a keyblade just as well as roxas and sora

that keyblade is called Destiny Place its only seen in one scene in Kingdom Hearts II, but in Kingdom Hearts Birth by Sleep, Kairi gives flowers to Aqua during their encounter, unlocking the Destiny Place for Aqua.

It turns out that the complete reason why Kairi could wield it was because she had a strong heart and she touched Aqua's Keyblade.

Kairi can use a keyblade because she gained the keyblade kinda the same riku did.

Spoiler Alert!!!
If you have played KHBBS, you'll remember that in Terra's storyline Terra handed 5 year old Riku his keyblade and 10-11 years later Riku is able to use a keyblade of his own. In Aqua's storyline in Radiant Garden Aqua protects Kairi from some Unversed and when Kairi runs to Aqua for protection, Kairi touches Aqua's keyblade.

So I'd say if someone touches a keyblade and the keyblade chooses them, then that person can use a keyblade of thier own. Sorry if I confused anyone.

The reason Kairi was able to wield a Keyblade is that she touched Aqua's Keyblade in Kingdom Hearts Birth by Sleep, which initiated a Keyblade Inheritance Ceremony.

her heart is stong like sora

I think It's just that anyone can use a keyblade. It's just that only sora can close the gates to the hearts of the worlds. In the first one riku used one. King mickey can use one. That's just my opinion though.

i think its because of 3 things, because she is the queen of hearts, she has a connetion to Sora, and she gave half of her heart to Sora.

kairi can use thekeyblade because she has a stron heart and has no darkness

play birth by sleep and get'cha self educated!

in BBS when she attacked by unversed, she ran to Aqua and hold Aqua's keyblade.

Kairi is able to wield a Keyblade because she touched Aqua's Keyblade when she was a child, initiating a Keyblade Inheritance Ceremony. [Kingdom Hearts Birth by Sleep]

What everyone else said about Kairi touching Aqua's Keyblade. I agree with them.

I was always under the impression that since sora carried kairis heart with him that she too began to have the ability. I could be wrong there are a few spots in the kingdom hearts timeline that I'm unfamiliar with, but that's what I feel to be right.

Gonna clear a lot of wrong answers I have read up, Kairi is able to use the keyblade because she touched Aqua's in Birth By Sleep and it was later awakened due to Riku lending her one, and as it happened with Roxas and Xion when he let her borrow it, she was able to awaken her abilities she got from Aqua. She can't summon it or at least it isn't revealed, just that in the secret ending for Dream Drop Distance she is summoned to begin training with her keyblade so she can master it. Now for the debunking of other posts I have seen. Aqua didn't actually mean to give her the keyblade, it was an accident since she was pure of heart and she touched Aqua's keyblade, like what Terra did with Riku, but completely accidental. Xion and Kairi have no connection, Xion is merely a puppet made of the memories stolen from Sora at Castle Oblivion during Chain of Memories. It is never revealed or mentioned that just because you are pure of heart like the 7 princesses of light, doesn't mean you can wield a keyblade, though it is safe to assume not, Kairi is just a special case due to her heart being so pure and she so happened to touch a Keyblade, so if a princess of heart were to touch a keyblade it might awaken the power.

In birth by sleep kairi came into contact with aqua's keyblade and it is shown that sora only has his keyblade because he touched rikus. This is revealed later in the story when riku took back the keyblade. Riku also gained the ability to wield the keyblade from touching Terra's keyblade.