After talking to the Archer at Fort Rimitz, we get the Overworld Quest "Building an Army". To complete the quest, we need to recruit mercenaries in Unicorn Overworld. Completing the task will earn you Renown, but that's not all. As the Archer said, they will work for the cause if you pay them. What that means is that you can assign them to your units up to a certain limit. So if you haven't hired any yet, scroll down and follow the steps to get some.
When character recruitment fails (Unicorn Overlord eggshells in your omelet)
Recruiting characters can be time-consuming, as there are conditions to fulfill. On the other hand, you can easily access and use them. Moreover, you can even customize those mercenaries, including their growth types and class.
Players can hire or recruit mercenaries from forts in exchange for honor. 64 is the limit for hiring. Each fort has limited class types, but not all are exclusive to a single fort. For example, Fort Rimitz has Housecarl, Hunter, and Thief. Fort Herstann has Wyvern Knight, Pritzlasse Fortress has Legionnaire, and so on. As you continue to complete the Liberation Quest, you will unlock different classes of mercenaries with the forts.
Just like the characters, make a place for them in the unit formation and check the list of available members. This time, find the mercenary you hired. Add him and use him. If you don't have any empty units, select Expand Units from a fortress and buy one with Honor.