Instructions
Play through Will's story line and get a rank of "C" or better on every mission. Each mission's objective is different, so you must listen to NiGHTS to learn how to complete each one. For example, some missions require you to collect a certain number of Nightopians within a given time limit. To get a rank of "C," you simply need to complete the objective before time runs out. The faster you complete it, the higher your score.
Play through Helen's story line and get a rank of "C" or better on every mission. As with Will's story, Helen must complete a variety of missions given by NiGHTS throughout the game. Follow her instructions to get the requisite rank for each mission. This includes boss fights.
Return to Will's Dream after you have completed the game with a "C" rank or higher with both characters. Go into the doors to Nightopia at the Dream Gate and select the "BellBridge" mission when you are prompted at the Mission Select screen. Repeat the final mission of the game to fight Wizeman again.
Go to Helen's Dream once you have beaten Wizeman with Will for the second time. Go through the doors to Nightopia and select the BellBridge mission again to fight Wizeman with Helen. When you beat Wizeman for the second time with both Helen and Will, you are awarded the Yellow Ideya.