Luigiis there if you want to switch to him during the level and only that level. if you get the star as luigi it will become a silverstar on the star checklist and won't be added to your star ammount. afterwards when you are back to the starship luigi falls asleep and mario hops back in but if your playing the american version (like me) then you will just be mario out of no where. i have 121 stars and 100% done. trust me. (after completing the game luigi's side of the adventure is avalible.)

Ghost Luigi serves no purpose in the overall completion. He is just programmed to show you the path to things like comet coins or hidden stars. You never have to activate him if you don't want.

He's there to show you through the level if you're having trouble, suddenly. You can also challenge yourself to beat your ghost to the star.

Actually, It's the ghost for the level. You can race it if you want. He isn't there to show you the way (shadow person shows you the way)

ghost luigi is there to guide you where the COMET MEDALS
are in the level

You unlock Ghost Luigi for beating a level as Luigi. It will guide you to hidden 1-Ups, Comet Coins, or even shortcuts to complete a level faster.