April O'Neil, Human Element

Legendary Creature — Human Detective

Whenever a player casts an artifact, instant, or sorcery spell, you create a Mutagen token. (It's an artifact with ", , Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")

"If you steal mutagen that doesn't exist from aliens that also don't exist, it's not really theft. More like creative writing!"