Meriones's bonus stats:

Increases the stats granted by a full set of Meriones's equipment by 1.5 times while in battle.

This means that the set bonus of a full equipment set will increase by 1.5 when the Hero is in battle.

An equipment set is considered "full" when you've collected all six unique items from the set. The pieces of equipment may differ in quality, but they still need to be part of the same equipment set. E.g., if you combine items from the "Guardsmen's Courage" and "Specialists' Authority" sets, the bonus will not work.

Tip. Equipping Meriones with gear that you wouldn't normally take into battle makes little sense.