Months after Sora's 'disappearance,' (End of KH3) Riku begins having dreams about a girl trapped in the Realm of Darkness. He asks his friends what it is he should do about it before doing research to find out if she's real or not. When Mickey and Aqua both begin receiving telepathic callings from the girl along with memories of her life, Mickey and Riku decide it's time to dive into the Realm of Darkness in search of her. Once in the Realm of Darkness, Riku makes a better connection and is able to learn that she's been drowning in darkness for many years after the sudden loss to all her friends and family being turned into Heartless and her world being engulfed in darkness. Unable to escape, she managed to make first contact with Riku before he first escaped the Realm of Darkness, pleading with him, to one day return for her when he was strong enough to face away the darkness that she couldn't. Since he's a Keyblade Master, he hopes he is strong enough to be able to find her and return her to the realm of light. The only thing he didn't count on, was falling in love with the girl she used to before falling to darkness, the girl she could one day become again.