Kaheera, the Orphanguard

Of the ten companions in the cycle, this is the one whose deckbuilding tax barely registers as one. The others demanded structural sacrifices: even-only spell costs, no repeated names, a deck of only two or fewer of each card. Kaheera asks only that every creature in your starting deck belong to one of five extremely common types (Cat, Elemental, Nightmare, Dinosaur, or Beast), and for decks that lean on removal, planeswalkers, and a handful of on-type threats, that restriction is close to free. So it became a fixture in decks with no interest in the tribal payoff printed on the card itself: the +1/+1 and vigilance anthem was incidental, and a guaranteed extra card waiting outside the game was what mattered. The rider is genuine when you do build around it, turning qualifying creatures into a wider, more attack-friendly board, but Kaheera's lasting significance is as the companion whose entry price could be waved through by a deck already built the way it wanted to be. It reframed what the mechanic could cost a player: not a rebuild, but a checkbox.

The errata that changed how companions enter play, requiring to move the card from outside the game into your hand at sorcery speed, applied to Kaheera as much as to the companions whose demands were actually painful. The tempo penalty landed hardest on the one whose restriction had been free, which was precisely the point of the fix.