Nardole, Resourceful Cyborg

The tension this design sits on top of is that undying and a counter-scaling mana ability want opposite things. The mana ability rewards a stockpile of counters, adding blue for each one; undying only fires if the creature dies with no +1/+1 counters on it. So the interesting state is the clean one. While the body carries no +1/+1 counters, it dies and comes straight back with a +1/+1 counter attached, and that returning counter is itself both a bigger body and another point of blue mana. But undying is a one-shot in a vacuum: the counter it grants is exactly what disqualifies the next trigger. To recur again you need an outside effect to strip the counter off, which is where the puzzle actually lives. Left alone, this is a single free rebound that permanently upgrades into a slightly larger creature and a slightly better mana source; loop it with counter-removal and it becomes a repeatable engine that refills its own fuel each cycle. The mana it produces is walled off to noncreature spells, so the design points toward a spell-heavy or artifact-leaning shell rather than a creature curve. Doctor's companion is the frame that gives it a home, letting it ride alongside a Doctor commander and bankroll a blue spell suite from the command zone. The whole card lives inside undying's blank-slate clause fighting a mana economy that only pays when the slate is not blank.