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.
Magic Elixir
Calvin's invisible form first appeared in the 8/28 through 8/29 1986 story arc. There, Calvin had rendered himself invisible with a magic elixir, 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 Calvin thought he could actually get away with his theft attempt, he seemed to believe his magic elixir worked and he was actually invisible.
School report
Calvin In Invisible Form reappeared in a much later strip as an exhibit for one of Calvin's outlandish Show and Tells at school. His performance complete, Calvin intended to leave the classroom, unstoppable as long as he was invisible.
Once more, Calvin seemed convinced of his alter ego's reality, pretending that his condition was real even when Miss Wormwood angrily picked him up and brought him back to his seat.