There was no body to under that grave but the implication was the same. The Keyblade Master, the savior of the worlds was gone. But to Riku it meant something greater, he would never get the chance to tell Sora how he felt. Now as he stood before the ornately carved tombstone the gravity of the whole ordeal crashed around him. Sora was his world. Everything that he had done in the past even the stuff he wasn't to proud of revolved around the spiky haired brunette. His whole fall into darkness had been because he wanted to prove that he was the stronger one, to better himself in Sora's eyes, but in the end it was still Sora that had proved to be stronger. After that Riku devoted his entire life to helping Sora on his journey in an effort to atone for his past sins.
When Diz told him that finding Roxas might help Sora wake up he went without a moments thought, he even cloaked himself in darkness all in the hopes of reviving him. After he awoke Riku watched from the shadows giving clues when needed, helping whenever he could. Even though the thought of letting Sora see him in his unsightly condition terrified him, he did not hesitate to come to his aid, and when he finally saw Sora he could not help but feel a unsettling surge of joy even though Sora saw him as an enemy.
He was torn from his musing by a distant roll of thunder. He looked up at the dark sky just as the rain began to fall. A sad smile tugged at his lips "Even the sky is crying for you Sora." he whispered as it began to rain harder. "Riku" he turned to see Kairi standing behind him an umbrella in her hand, "It's time to go." she continued gently. He nodded mutely looking at the grave last time
before, he turned and walked away heart heavy and fresh tears burning down his cheeks only to mingle with the rain.