Companion is an ability word, a label with no rules meaning of its own. It marks abilities that all impose a deck-building condition on your starting deck. The oracle text shows the pattern: 'Your starting deck contains only nostalgic cards,' 'Each nonland card in your starting deck shares a card type,' 'Each nonland card in your starting deck has a different name,' or 'Your starting deck contains only cards with even mana values.' When you meet the condition, the reminder text lets you put that card into your hand from outside the game for {3} as a sorcery.
In play, Companion turns deck construction into a puzzle. The restriction is always on, so every slot must satisfy it, but the reward is a guaranteed extra card you can access later. That trade-off shapes mulligans and sideboarding, and it means the deck's identity is defined by the condition rather than the card's own text.
