Galvanic Alchemist

Soulbond only pairs two creatures together; it grants nothing on its own. The engine here lives in the separate static ability, which hands both members of the pair an untap valve: pay the cost, ready either creature, repeat as your mana allows. The 1/4 body is the tell about intent. This is not a beater; it is a battery wired to something that does work when it taps. Point the pairing at a creature whose tap is the payoff (a mana producer, a fight or ping effect, a tapper of its own) and the untap clause converts a once-per-turn activation into a per-mana one. The tension is that the valve costs blue to spin, so it competes for the same resource that fuels whatever you are untapping; the loop is real but throttled by how much mana you can pour in. The fragility is structural: soulbond holds only while you control both halves, so the pairing dissolves the instant the partner leaves play, and the untap ability vanishes with it (it was granted by the static ability, which now finds no pair to grant to). What remains is a plain 1/4 with no engine at all. That makes it a build-around with a narrow center of gravity, an enabler that asks you to already own the activated ability worth doubling before it earns a slot.