Comprehensive Rules
702.139a
Companion is a keyword ability that functions outside the game. Itâs written as âCompanionâ[Condition].â Before the game begins, you may reveal one card you own from outside the game with a companion ability whose condition is fulfilled by your starting deck. (See rule 103.2b.) Once during the game, any time you have priority and the stack is empty, but only during a main phase of your turn, you may pay and put that card into your hand. This is a special action that doesnât use the stack (see rule 116.2g). This is a change from previous rules.