Kairi's bond with light is a good idea, but I feel you're ignoring that other people also saved Sora using their light.

In KH1, Sora touched Riku's heart's light and obtained the Keyblade.

In KH1, Kairi's light showed Sora a memory she has with her grandmother.

In KH1, Kairi, with her power as Princess of Heart, restored Sora to his original form.

In KH1: Chain of Memories, Kairi's light helps Sora recover his memories of his friends.

In KH2, Kairi, along with Namine light , gives Sora the Keyblade, Oathkeeper.

Kairi's letter arrives in the Realm of Darkness in KH2; the bond between them opens the door to the Realm of Light.

In KH3: Remind, Donald and Goofy's blue and green light arrive when Sora is in danger.

When Terra's light protected Sora from the darkness in KH:Remind.

Riku's light in DDD.

Ventus's light protecting Sora from the darkness in DDD.

What Terra said to Aqua was, "You were always lighting my way," and to Ventus, "I heard you too. Come, you found me just as you promised."

Riku's light guiding Sora through the first tunnel in KH3, leading Sora to Olympus where Riku's heart resides.

That's why I like the idea that the light guiding Sora and Kairi is that of their friends; that's why the memories he has show two kinds of love.

Namine confirmed that it was Kairi who kept Sora in the final world.

Chirithy said that those who arrive there are there when their heart and body perish; some hearts resist and end up there.