You will need to obtain the below six items and then finish Olive’s case Deskwork…? in File 12 to obtain the Lappy Costume.
1. Lappy Badge – Finish the following blue side cases during File 03 in order to receive the badge:
2. Lappy Arms – During the blue side case Lappy’s Lament in File 05, you can obtain the arms after beating Jin in the fight by getting his mood to 100.
3. Lappy Wisdom – Finish the Lappy’s Regard side case during File 07 to receive the wisdom.
4. Lappy Legs – In File 08 talk to Marie and agree to help her test out Lappy’s Balloon Bash mini-game. In order to receive the legs, you just need to play the game once.
5. Lappy Body – Finish the Lappy’s Ultra Quiz side case during File 08 to receive the body.
6. Lappy Head – Purchase the item from Brenda during File 12 for 100,000G.