Unhasty

From the Comprehensive Rules 4.0:
The Free Peoples player cannot assign a character with this keyword to a skirmish unless it is at its home site (see ally) or when a Gandalf-Culture card allows it do so.
Shadow players may assign any leftover unassigned minions to unhasty companions.
See also Assignment phase.

One of the Gandalf cards to let your Ent Ally skirmish is Stump and Bramble.

List of all Characters with the Unhasty Keyword