—In-game description, Rekka no Ken
The Chest Key (宝の鍵 Takara no kagi, lit. Treasure Key) is a key that recurrently appears across the Fire Emblem Series. The Chest Key is, in essence, a skeleton key that is specifically designed to allow its holder to bypass all locks imposed on Chests and thereafter retrieve the Items contained within. Chest Keys feature varying Durability values across the series, and can be used as a means to substitute Thieves, units who are able to unlock both Doors and chests without the aid of keys.
Chest Keys are commonly dropped alongside Door Keys, items that are specifically used to unlock doors. In such situations, the doors in question usually function as barriers to rooms containing chests.
In the Archanea Series of Fire Emblem, following the acquisition of the Fire Emblem, Marth can be used to substitute the chest-raiding role that is otherwise filled by both the Chest Key and Thieves.