I remember when Knuckles was first playable back in Sonic & Knuckles and Sonic 3 & Knuckles. His character was clearly established as the guardian of a giant emerald called 'The Master Emerald.' This concept would carry through into the 3D era. In Sonic Adventure, the Master Emerald was destroyed, sending the home of the emerald, Angel Island, falling into the sea. Knuckles spent the game finding the missing shards emerald and restoring them, thus returning Angel Island to the sky. In SA2, the same thing occurs. However, it doesn't show Angel Island at all once the emerald is broken. I just assumed that Angel Island fell into the sea again with no damage done to it. When Sonic Heroes, Shadow the Hedgehog, and 06 came around, however, Knuckles' role s the guardian of the Master Emerald seemed to have been completely forgotten. I mean, unless Knuckles has someone else guarding the emerald while he's out screwing around with Sonic and Tails. As the years went by, SEGA suddenly remembered Knuckles' role as the guardian. He didn't appear in Sonic Unleashed or Sonic Colors, so that's just what I assume.