The Most Powerful Old 6 System

On the first day of the apocalypse, I bound myself to the "Strongest Sixth System". Others fight zombies, but I specialize in stealing their pants. Others built a base, but I broke into the base safe overnight. Relying on my humble growth, I became the richest sixth child in the last days. Until the system releases the ultimate mission: "Steal the Zombie King's underwear and laugh at him in front of his face." I lurked for three months and finally succeeded, only to meet the zombie king's scarlet eyes. He spoke slowly: "What you are holding is my last dignity."