As with most new sets, Ikoria comes with some new abilities and keywords. While Mutate has been getting the lions-share of the attention (and deservedly so, if for no other reason than complexity), Companion is attracting a few looks as well.
How does Companion work, anyway? Companion is a great way to lower the randomness of gameplay. Before the game starts, you can declare one creature with Companion in your sideboard as your designated Companion for the game. If you do, it starts face-up outside the game, similar to a Commander. Also, similar to a Commander, you may cast your companion from outside the game any time you could cast it from your hand (i.e when you can pay its mana cost and the stack is empty on your turn). Sounds great, right? Why wouldn’t you run a Companion every game?
Well, much like Commander, Companions come with deck building restrictions. Each companion has its own special restriction. While Keruga, the Macrosage won’t be your companion unless you only run things that cost 3 or more, Obosh, the Preypiercer can’t even. Some restrictions might seem like fun build arounds (Zirda, the Dawnwaker only wants to play with permanents with activated abilities), but others might seem overly stifling (Lutri, the Spellchaser will double all your instants and sorceries, but is it worth it if you can only run one of each nonland card in your deck?).
Unlike Commander, once you’ve cast your Companion, they act in all other respects exactly like a creature cast from your deck. When they die, they go to your graveyard. There is no way to continously replay them like a Commander (well, unless your deck is somehow built to continously replay a creature). They retain no special status to damage or prevent them from being shuffled into your deck or exiled. And you can always just run one in your maindeck if you don’t need to cast it on turn every game.
Ultimately, whether it is worth the cost to run a Companion is going to depend on the deck and what it gets out of it, but these creatures are sure to inspire some interesting decks. Personally, I can’t wait to see the look on Niv Mizzet Reborn’s face when he finally gets a look at Jegantha, the Wellspring!