This wasn't really made clear in any of the publications or gatherer rulings, so does anyone happen to know: in a best of 3 game, are you required to follow the companion deck requirement in only the first match or all matches? In other words, are you allowed to sideboard in cards that violate the companion requirement but still keep that creature as your companion?
In other words, are you allowed to sideboard in cards that violate the companion requirement but still keep that creature as your companion?
702.138. Companion
702.138a 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 103.1b.) If you do, once during that game, you may play that card from outside the
game.
702.138b 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.
For example, suppose one of the cards in your sideboard is Keruga, the Macrosage with "Companion — Your starting deck contains only cards with converted mana cost 3 or greater and land cards." If you declare it as your companion for game 2 (again), your starting deck for game 2 had better contain only cards with converted mana cost 3 or greater and land cards. If you sideboard in an Aegis Turtle for game 2, don't declare Keruga as your companion for game 2.
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Why bother with mere rulings when so many answers can be found in the Rules?
Declaring a companion is something you need to do for each single game in the match.
If you are going to use the same companion for each game, you need to declare it again for Game 2, and again for Game 3.
You can also have no companion for game 1, then declare one for game 2. Or have one for game 1, but none for game 2. Or have different companions each game!
You declare your companion, if any, after sideboarding. Your deck after sideboarding need to follow the companion's requirements.
Ok, the release notes explicitly say my Commander has to meet the requirements, but that comp rule is confusing.
Your companion is not one of your one hundred cards. Your commander is.
103. Starting the Game
103.1. At the start of a game, each player shuffles their deck so that the cards are in a random order.
Each player may then shuffle or cut their opponents’ decks. The players’ decks become their
libraries.
103.1a If a player is using a sideboard (see rule 100.4) or cards being represented by checklist cards
(see rule 713), those cards are set aside before shuffling.
103.1b If a player wishes to reveal a card with a companion ability that they own from outside the
game, they may do so after setting aside their sideboard. A player may reveal no more than one
card this way, and may do so only if their deck fulfills the condition of that card’s companion
ability. (See rule 702.138, “Companion.”)
103.1c In a Commander game, each player puts their commander from their deck face up into the
command zone after having the opportunity to reveal a card with a companion ability and before
shuffling. See rule 903.6.
103.1d In a Conspiracy Draft game, each player puts any number of conspiracy cards from their
sideboard into the command zone before shuffling. See rule 905.4.
The companion condition is applied while your "deck" still has all one hundred cards -- including the commander.
If you want to reveal a card with a companion ability as "your companion", all the cards in your deck — including your commander if any (C.R. 903.3) — have to comply with that companion ability's deck-building restriction. Thus, for example, if you want to reveal Gyruda, Doom of Depths this way in a Commander game, your commander must have an even converted mana cost just as well as the other cards in your deck must.
In a Commander game, however, you can't bring a card into the game from outside it (whether by the companion ability or otherwise) if—
the card has the same name as a card in your starting deck (including your commander [C.R. 903.3]) or a card you own in the current game, or
the card's color identity includes colors outside your commander's color identity
(C.R. 903.11).
As a result, for example, in a Commander game you can't bring Gyruda, Doom of Depths into the game from outside it (whether by the companion ability or otherwise and even if Gyruda is "your companion") unless—
your commander's color identity includes blue and black (C.R. 903.4),
your starting deck's commander was not named Gyruda, Doom of Depths,
your starting deck otherwise had no cards named Gyruda, Doom of Depths, and
you don't own a card named Gyruda, Doom of Depths in the current game.
As a further result, since currently nothing in the game refers to "your companion" in a similar way as some cards refer to "your commander", there is currently little if any practical benefit to revealing Gyruda as "your companion" in a Commander game if any of those four conditions is not met.
EDIT (Jun. 20): Edited to conform to updated comprehensive rules for June 2020.
EDIT (Jul. 23, 2021): Edited to conform to rule update with Adventures in the Forgotten Realms.
Every writeup I've seen for Companion describes that as a per-game declaration: https://magic.wizards.com/en/articles/archive/feature/ikoria-lair-behemoths-mechanics-2020-04-02
For example, suppose one of the cards in your sideboard is Keruga, the Macrosage with "Companion — Your starting deck contains only cards with converted mana cost 3 or greater and land cards." If you declare it as your companion for game 2 (again), your starting deck for game 2 had better contain only cards with converted mana cost 3 or greater and land cards. If you sideboard in an Aegis Turtle for game 2, don't declare Keruga as your companion for game 2.
If you are going to use the same companion for each game, you need to declare it again for Game 2, and again for Game 3.
You can also have no companion for game 1, then declare one for game 2. Or have one for game 1, but none for game 2. Or have different companions each game!
You declare your companion, if any, after sideboarding. Your deck after sideboarding need to follow the companion's requirements.
The companion condition is applied while your "deck" still has all one hundred cards -- including the commander.
In a Commander game, however, you can't bring a card into the game from outside it (whether by the companion ability or otherwise) if—
As a result, for example, in a Commander game you can't bring Gyruda, Doom of Depths into the game from outside it (whether by the companion ability or otherwise and even if Gyruda is "your companion") unless—
EDIT (Jun. 20): Edited to conform to updated comprehensive rules for June 2020.
EDIT (Jul. 23, 2021): Edited to conform to rule update with Adventures in the Forgotten Realms.