Life Matrix — ruling 1

Once the counter is placed on the creature, the ability to remove it can only be activated by the player who controls the creature.

Life Matrix : règles, fonctionnement et interactions (MTG)

Life Matrix is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

, : Put a matrix counter on target creature and that creature gains "Remove a matrix counter from this creature: Regenerate this creature." Activate only during your upkeep.

How it works : Life Matrix

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
Once the counter is placed on the creature, the ability to remove it can only be activated by the player who controls the creature.
No other card is named in this ruling.