What does a 'Character card from outside the game' mean?
[Source: WotC] 
When a card refers to “a Character card from outside the game”, those cards from outside the game must be in the possession of the player at the beginning of the match.
Character cards from outside of the game do not count as part of players’ teams, decks, and sideboards, but must be referenced by other cards that are part of a player’s team, deck, or sideboard
At the end of each game, any Character cards from outside the game that were introduced during play must be removed from players’ teams, decks, and sideboards.
Character cards from outside the game must be legal within the event format.
When a card refers to “a Battle card from outside the game”, those cards from outside the game must be in player’s sideboard at the time of using the effect.
At the end of each game any Battle cards from outside the game that were introduced during play are returned to players’ sideboards.
Battle cards from outside the game come from the sideboard and adhere to all sideboard rules.
