Keruga, the Macrosage — ruling 1

Once you put your companion into your hand, it behaves like any other card you’ve brought into the game. For example, if it’s countered or destroyed, it’s put into your graveyard, remaining in the game.

Keruga, the Macrosage — ruling 2

Wizards of the Coast has issued functional errata for the Companion mechanic. Instead of casting companions from outside the game: Once per game, any time you could cast a sorcery (during your main phase when the stack is empty), you can pay {3} to put your companion from your sideboard into your hand. This is a special action, not an activated ability. It happens immediately and can’t be responded to. It can’t be countered or stopped by cards like Phyrexian Revoker. For more information please see https://magic.wizards.com/en/articles/archive/news/june-1-2020-banned-and-restricted-announcement

Keruga, the Macrosage — ruling 3

If you reveal a companion outside the game, for as long as it remains there, you may pay {3} any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and behaves like any other card you've brought into the game. For example, if it's discarded, countered, or destroyed, it's put into your graveyard, remaining in the game. This is a change from previous rules.

Keruga, the Macrosage — ruling 4

Paying {3} to put your companion into your hand is a special action. It doesn't use the stack and players can't respond to it. Once you take this action, you may cast that card if it's legal to do so before any other player can take actions.

Keruga, the Macrosage — ruling 5

If a card in a player's deck or a permanent on the battlefield has {X} in its mana cost, X is considered to be 0.

Keruga, the Macrosage — ruling 6

The mana value of a token that isn't a copy of another object is 0. A token that is a copy of another object has the same mana cost as that object.

Keruga, the Macrosage — ruling 7

Your companion begins the game outside the game. In tournament play, this means your sideboard. In casual play, it's simply a card you own that's not in your starting deck.

Keruga, the Macrosage — ruling 8

Before shuffling your deck to become your library, you may reveal one card from outside the game to be your companion if your starting deck meets the requirements of the companion ability. You can't reveal more than one. It remains revealed outside the game as the game begins.

Keruga, the Macrosage : règles, fonctionnement et interactions (MTG)

Keruga, the Macrosage is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Companion — Your starting deck contains only cards with mana value 3 or greater and land cards. (If this card is your chosen companion, you may put it into your hand from outside the game for as a sorcery.) When Keruga enters, draw a card for each other permanent you control with mana value 3 or greater.

How it works : Keruga, the Macrosage

Original English text — Wizards of the Coast / Scryfall

#1June 1, 2020· Scryfall
Once you put your companion into your hand, it behaves like any other card you’ve brought into the game. For example, if it’s countered or destroyed, it’s put into your graveyard, remaining in the game.
No other card is named in this ruling.
#2June 1, 2020· Scryfall
Wizards of the Coast has issued functional errata for the Companion mechanic. Instead of casting companions from outside the game: Once per game, any time you could cast a sorcery (during your main phase when the stack is empty), you can pay to put your companion from your sideboard into your hand. This is a special action, not an activated ability. It happens immediately and can’t be responded to. It can’t be countered or stopped by cards like Phyrexian Revoker. For more information please see https://magic.wizards.com/en/articles/archive/news/june-1-2020-banned-and-restricted-announcement
No other card is named in this ruling.
#3June 1, 2020· Wizards of the Coast
If you reveal a companion outside the game, for as long as it remains there, you may pay any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and behaves like any other card you've brought into the game. For example, if it's discarded, countered, or destroyed, it's put into your graveyard, remaining in the game. This is a change from previous rules.
No other card is named in this ruling.
#4June 1, 2020· Wizards of the Coast
Paying to put your companion into your hand is a special action. It doesn't use the stack and players can't respond to it. Once you take this action, you may cast that card if it's legal to do so before any other player can take actions.
No other card is named in this ruling.
#5April 17, 2020· Wizards of the Coast
If a card in a player's deck or a permanent on the battlefield has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#6April 17, 2020· Wizards of the Coast
The mana value of a token that isn't a copy of another object is 0. A token that is a copy of another object has the same mana cost as that object.
No other card is named in this ruling.
#7April 17, 2020· Wizards of the Coast
Your companion begins the game outside the game. In tournament play, this means your sideboard. In casual play, it's simply a card you own that's not in your starting deck.
No other card is named in this ruling.
#8April 17, 2020· Wizards of the Coast
Before shuffling your deck to become your library, you may reveal one card from outside the game to be your companion if your starting deck meets the requirements of the companion ability. You can't reveal more than one. It remains revealed outside the game as the game begins.
No other card is named in this ruling.
#9April 17, 2020· Wizards of the Coast
The requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard.
No other card is named in this ruling.
#10April 17, 2020· Wizards of the Coast
If more than one player wishes to reveal a companion, the starting player does so first, and players proceed in turn order. Once a player has chosen not to reveal a companion, that player can't change their mind.
No other card is named in this ruling.
#11April 17, 2020· Wizards of the Coast
If you reveal a companion outside the game, for as long as it remains there, you may pay any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and behaves like any other card you've brought into the game. For example, if it's discard, countered, or destroyed, it's put into your graveyard, remaining in the game. This is a change from previous rules.
No other card is named in this ruling.
#12April 17, 2020· Wizards of the Coast
Paying to put your companion into your hand is a special action. It doesn't use the stack and players can't respond to it. Once you take this action, you may cast that card if it's legal to do so before any other player can take actions.
No other card is named in this ruling.
#13April 17, 2020· Wizards of the Coast
The companion's other abilities apply only if the creature is on the battlefield. They have no effect while the companion is outside the game.
No other card is named in this ruling.
#14April 17, 2020· Wizards of the Coast
The companion ability has no effect if the card is in your starting deck and creates no restriction on putting a card with a companion ability into your starting deck. For example, Zirda may be in your starting deck even if your other permanent cards don't all have activated abilities.
No other card is named in this ruling.
#15April 17, 2020· Wizards of the Coast
You may have one companion in the Commander variant. Your deck, including your commander, must meet its companion requirement. Your companion is not one of your one hundred cards.
No other card is named in this ruling.