Garrison is unlocked at the start of chapter 7 in campaign.
Players should prioritize getting Rem (blue hair girl with mace), and later Shuna if they have spare resources.
#Garrison
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Hero Garrison is basically a Rent-A-Dimensional feature with unlimited use of the Dimensional everywhere, unlike mercenaries that can only be used once in selected game modes each per week. It's almost like owning the Dimensional, except that you have to keep reapplying for access every month, and you cannot merc it out or let someone else Garrison it from you. You'll still have to work on SI and furniture (your investment into the hero remains after each rental is over), and decide who to link it to.
Friends who garrison out their hero will receive 40 companion points and can garrison out the same hero 5 times per month.
Go to Friends > Garrisoned.
Use Guild/Hero/Lab coins to exchange for Dimensional stones.
Use 100 Dimensional stones to rent 1 Dimensional for 1 month.
After renting a particular Dimensional hero for 6 months in total, you no longer need to pay 100 Dimensional stones to rent that particular hero each month from then on; it becomes free to rent. You can rent that particular hero from different players each time and it will still count towards the 6 times.