With Ariel and Triton both under Ursula's poison, Kairi was the only one left to fight against her. Outside the castle walls, the battle between Triton's warriors and Ursula's armies was proving to be relative stalemate. Many of Triton's followers had fallen, but so had Ursula's. As Ariel and Triton lied on the ground, Kairi pointed her keyblade at Ursula.
Kairi: It's time to finish you off for good.
Ursula: Hahaha! You think you're the one who'll be able to do that? Keep dreaming dollface. You couldn't even beat Lucifer and I am Ursula, ruler of the seas!
Kairi was tired of talking, she lunged right into battle swinging her keyblade at Ursula, but Ursula simply blocked with her keyblade and pushed Kairi backwards. Ursula then swum upwards and used her tentacles to turn her body into somewhat of a spear. Spinning around quickly, she flew downward at Kairi, attempting to plow through her chest, but Kairi swum to the left and swung her keyblade around at Ursula who blocked it with a tentacle.
Kairi continued attacking sporadically as she tried to get a hit on Ursula, but she was having no success. Her attacks were either being blocked by tentacles, blocked by Ursula's keyblade, or avoided altogether. After a good number of unsuccessful attacks, Ursula got tired of playing around and stabbed her keyblade into Kairi's chest.
Poison began seeping into Kairi's body as she found herself drifting away. The world around her was all swirls, as she went into her mind. The next thing she knew, she was standing in an empty space looking at several images of herself. The images faded away one by one until she was enveloped in total darkness. She was about to pass out when she heard a familiar voice.
???: Don't give up now Kairi. There are too many people depending on you. Sora needs you, Riku needs you, we all need you. That's why you are one of the seven guardians of the light Kairi.
Kairi: How did I get chosen for this duty? How do you know I'm cut out for it?
???: Stop doubting yourself Kairi. You are better than that. You have always lunged forward believing that light would prevail.
Kairi: But I've done all I can. I'm just not on par with these other warriors.
???: You are, you just haven't found yourself yet. Kairi, look closely at your surroundings, you are still there.
Kairi let out a deep breath and looked around to see that she was still in the same area with many versions of herself standing in the light. Standing in front of her was the king, Mickey who had been motivating her.
Kairi: I see it now. I really am still here. These versions of me are all a part of my heart. I just have to believe in myself. I have to find the warrior inside of me.
With that, Kairi looked to her left to see a tough looking version of herself. Kairi walked over to her and was surprised when the image she saw changed. Instead of looking at herself, she was looking a cloaked girl with black hair.
Kairi: Who are you?
Xion: I am you, but I am not you. I represent the you in the heart and mind of Sora. I am your strength.
Mickey: Kairi, reach out to her. Take your new power.
Kairi gave Mickey a nod and reached out to Xion. Xion grabbed her hands back and Kairi absorbed her spirit. With that, she felt herself returning to the world of Atlantica as the poison excreted from her body. Her fin turned into legs as she was able to walk on the ocean floor and breathe underwater. Most importantly though, she was no longer carrying a keyblade. She was holding two.
Kairi: My keyblade is the Duo Reverse. When I reach my full power, it becomes two, both with even more power than the original. It restores my body to its state five minutes prior and allows me to continue battling. My time keyblade thrives on the power of magic, by inflicting me with poison magic, you gave me all the magic I needed to evolve.
Ursula: It doesn't matter. You still face Ursula, and your friends are still going to die.
Kairi: Let's see about that.
Kairi jumped quickly at Ursula and swung her keyblades at her. It was so fast that Ursula couldn't move. She was smacked between both keyblades as she started falling downward. Kairi kicked downward and stabbed Ursula in the chest with both blades. Ursula was pinned to the ground as Kairi pulled her blades out. She pointed one at Ursula as she pointed the other one away, the one she pointed away gathering magic.
Kairi: Now, free Ariel.
Ursula: Fine, but this isn't over Kairi. Next time we face off, I'll defeat you.
Ursula snapped her fingers and disappeared from sight. Afterwards, Ariel and Triton returned to normal and Ursula's army retreated, for now. Triton and Ariel hugged as the armies returned to the palace. Triton through a celebration party for Kairi and Ariel, thanking them for all their help. Kairi left immediately after, on her way to the next world. Ariel would return to the land above, but her story would not end there.
As for Ursula, she returned to her lair in anger, as she slammed her hands against the coral reef, causing them to collapse. Another Organization member arrived in her lair at that time.
???: Oh feisty. You know Ursula, I always thought you were a bit melodramatic. But this takes the cake.
Ursula: You? What are you doing here? Did you come to mock me again? You're a poor excuse for a member of the Organization.
???: Actually, I consider myself the greatest member. That said though, I didn't come to chat. I came to take something that I want.
???: Open wide!
With that, a glowing stream exited from Ursula's mouth into a small container that the Organization member was holding. Ursula tried to speak but she couldn't. She didn't understand.
???: Oh come on Ursula. Cat got your tongue? Maybe that's because I have your voice now.