Outfit

Outfits, also referred to as Skins and Costumes, are alternative, and often optional clothing that characters can wear, primialy serving as nothing more than aesthetic change.

Sonic Adventure
In Sonic Adventure, upon collecting all 130 emblems, a bonus Metal Sonic skin can be unlocked for Sonic,

Sonic Adventure 2
In Sonic Advenutre 2,  several costumes can be downloaded for all 6 main characters, including christmas-themed and halloween-themed costumes.

Sonic Rivals 2
Sonic Rivals 2 provides all 8 of the playable characters with 4 diffrent alternate costumes that can be unlocked by first meeting and completing certain challenges.

Sonic Lost World
When playing the Legend of Zelda Zone DLC stage in Sonic Lost World, Sonic will wear an outfit resembling that of Link's.