The Seven Guardians of Light, or "The Seven Lights" is small group of Keyblade wielders who are destined to oppose the Thirteen Darknesses and create the X-blade in the process.
During Dream Drop Distance, Master Xehanort revealed that he knew his attempts to create X-Blade through the reunion of Ventus and Vanitas was a hasty attempt due to his impatience and that he had always been searching for a true way to forge it. For this reason he enticed Maleficent into capturing the Princesses of Heart, with his Heartless Ansem overseeing her progress, while his Nobody Xemnas created Organization XIII so his splintered selves can obtain the Lights and Darknesses, respectively. But, as Ansem failed to possess Riku and Roxas returned to Sora, Xehanort's new organization would be one member short so he decided to use Sora. As Xehanort explained his plan to Mickey, in order to make sure the Seven Lights form, Mickey hypothesises that the current candidates for this role would be himself, Sora, Riku, Terra, Ventus, Aqua and another he didn't mention out loud as he realised who the Thirteen Darknesses were. Xehanort explains that while the King was right, Terra was unavailable as he was his vessle and Sora was about to become the Thirteenth member of his Organisation. This attempt, too, failed thanks to Lea, Donald and Goofy.
As the 13 Darknesses return back to their timelines while short one member, Xehanort tells his opponents that the battle between his Darknesses against their Lights will happen and they will meet again at the fated place. Because of this threat, fearing Xehanort would go after the Princesses of Heart if they did not go through with it, Yen Sid and Mickey resolve to gather the Seven Lights. After Riku awakens Sora and Yen Sid grants Riku the title of Master it is revealed that Lea can also wield the Keyblade, making him another potential member. The secret ending of Dream Drop Distance reveals that Yen Sid has Kairi brought before him to teach her to wield her Keyblade, likely making her the seventh Light.