MTG Keyword Rules — Comprehensive Rules Glossary

Companion is a keyword ability that functions outside the game. It’s written as “Companion—[Condition].” 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 {3} and put that card into your hand. This is a special action that doesn’t use the stack (see rule 116.2g). This is a change from previous rules.

If a companion ability refers to your starting deck, it refers to your deck after you’ve set aside any sideboard cards. In a Commander game, this is also before you’ve set aside your commander.

Once you take the special action and put the card with companion into your hand, it remains in the game until the game ends.

Cards can enter Commander games from outside the game via the companion special action.