You can buy an invisible effect in the Poptropica Store, but it will not work to hide you on Spy Island. Instead, you need the Chameleon Suit.
Spy Island
Get the Chameleon Suit from Dr. Spyglass (take an eye exam and pick the reverse symbols from the ones he points to). When you are standing still, you cannot be seen. This helps you get past the guards on the docks.
Wild West Island
You will need the old saddle that the cowgirl gives you in Rock Ridge, and you need to win the gold claim map in the Dos Cactos casino. Swap the saddle for an oil pan at the Trading Post, and pan for gold at the map location. Use the gold nugget to but the various concoctions from the peddler in Rock Ridge. You need the Transparency Tonic to slip into the outlaw hideout.
Only if you want to. It is not necessary.
Melle.Mareaa in mystery island
Nikola Tesla is in the room labeled "Nikola Tesla". You need to disguise yourself as a porter to get in.
To get past the guards in Counterfeit Island in Poptropica, you need to use the disguise. First, collect the necessary items to create a fake mustache and glasses. After equipping the disguise, approach the guards, and they will let you pass, thinking you are someone else. This allows you to explore further into the island and continue your quest.
Unless it is for sale at the Store, you would have to get one with the costumizer. (Did not try Shark Boy on Shark Tooth Island.)
The Old Lady is actually the goddess Athena! So she isn't really an old lady, that was just a disguise!
Planet disguise good. Some planet disguise better. Planet disguise by disguising.
No Disguise was created in 2007.
The word disguise is a noun. It can also be a verb as in (e.g.) to disguise your true feelings.
Who is Hannah Montana in disguise?
Disguise an insult as a compliment
That is the correct spelling of "disguise" (a mask or camouflage).