One of your allies can be appointed as a Substitute.
Enter the Diplomatic Corps and choose nickname of a player you want to be your Substitute in the drop-down list in the “Substitute” tab.

Substitute can optionally be given the access to the mail.

Substitute will be able to:
Substitute's online isn't considered as the owner's online.
Substitute's entrance prevents the riot, but doesn't prevent the account deletion due to a long inactivity of the owner.

Substitute can perform any actions the owner can, except for:

Substitute doesn't have access to the owner's information about Kingdoms. That's why he can't initiate missions.