Calvin In Invisible Form is a minor recurring alter ego of Calvin's. He appeared in a story arc and a weekday strip as an invisible version of Calvin with nefarious intentions.
8/28 through 8/29 1986 story arc
Calvin's invisible form first appeared in this 1986 story arc. There, Calvin had rendered himself invisible with a magic elixir, removed his still-visible clothes, and set out to commit shady deeds, undetected. The only such effort featured was his foiled, real-life attempt to steal cookies. Calvin's invisible form was analogous to his attempted cookie theft, as well as to Calvin's mother considering that Calvin "disappeared" whenever something was asked of him.
As Calvin thought he could actually get away with his theft attempt, he seemed to believe his magic elixir worked and he was actually invisible.