Phyrexian Walker

A free creature is a strange thing to balance, and the design answer here is to give you something that does almost nothing. Zero mana buys a 0/3 with no abilities: a body that puts no damage on the board and asks for nothing in return. That neutrality is the entire point. A creature spell that costs nothing still goes on the stack, still resolves, still counts toward a creature in play, and still leaves an artifact behind when it dies. This is one of a handful of free-to-cast bodies from the era that were never meant to be played for their stats; they exist as permanents other cards care about. Anything that asks for a creature, an artifact, a sacrifice, or a number of spells cast can take this for free and ask no questions. The 0/3 frame matters: a toughness of 3 means it survives more incidental damage than a 0/1 token would, so it sticks around as a blocker or a fodder reserve rather than evaporating. And because it lacks Defender, it can legally swing into combat, which turns it into a cheap chassis for anything that adds power from the outside: equipment, an anthem, an attack trigger that does not care how big the attacker is. This is enabler design at its most literal, a permanent whose worth is defined entirely by the cards played alongside it rather than by anything printed on its own face.