Okay so we know that Sora is in love with Kairi and she loves him. However, in the first game it is possible that Riku loved her more. I mean he always played around with Sora about the Paupu fruit, but Riku submerged himself in the darkness, captured the princesses and all that stuff just to get Kairi's heart back. Sora however didn't work as hard as Riku to get Kairi back. Kust like Riku said in the game. Also Kairi did like Riku and Sora so it was just childish play for their competition to win Kairi's heart. So Sora loved Kairi more throughout the series, but for the first game I believe that Riku cared about Kairi to the extent of changing his whole life.