Each character can only carry five items. If you acquire a sixth item, you must either surrender one item, or send something to the Transporter. There is one Transporter in Fire Emblem - a man named Merlinus. You must rescue Merlinus in Eliwood's Story, and he will join your party. You can then choose to deploy Merlinus during battle.

Merlinus can hold up to 100 items at a time. Once deployed in battle, your units can visit his tent to stock up on items, or they can purchase items at a shop and have them sent directly to Merlinus. Be careful, however, as enemy units can attack Merlinus and he has no way to defend himself. If he loses all HP, he will fold up his tent and leave the battlefield. If he survives the battle, he gains one level.