Bill Potts

The 2/4 body is doing quiet defensive work before the trigger ever matters: this is a creature built to survive being targeted, not to attack, and the toughness lets your spell resolve without combat math intruding. The reward comes with an unusual restriction: the spell or ability must target only Bill Potts and nothing else. That single-target clause governs the entire build. It closes the door on any spell that names two things, so the payoff belongs to a deck of pinpoint effects (a targeted pump, a lone-target burn spell, an activated ability aimed at exactly one creature) rather than sweeping value spells. The copy inherits a freedom the original was denied: you may choose new targets, so the second instance lands wherever you like while the first was forced onto Bill. The once-per-turn cap prevents the loop from spiraling and sets the card's rhythm, one doubling on each turn instead of a chain. The Doctor's companion clause shows how the design expects to be assembled, letting her partner with a Doctor and reach into a wider pool of single-target spells to feed the trigger. She reads as a support piece more than a finisher: the interesting decisions live in what you aim at her, knowing the copy will find somewhere better to go.