If you purchase Special Character 1 and 2, you can get Yukulf and Hargna in your party, after you've beat Gardino in the Closed Lands pt 1.
Note that having Yukulf and Hargna in your party does not change any part of the story! It's simply something to do for fun, and to give you some extra options for team formation.
You can get Yukulf in your party by going to see Garganbu in the Dwarves’ Village.
He comes with the same weapons, armor, and skills that he had when he joined your party in Zillballinay.
You can find Hargna in the Eastern Shadowlands, in front of the fountain next to Karin’s house.
She comes with the same weapons, armor, and skills that she had when she joined your party in the Granzalk Garrison Post.
Bonus Weapons and Changing Armor
After completing the Group Training in the Granzalk Garrison Post, you can buy new weapons (with new skills!) for Yukulf and Hargna in the Granzalk Garrison Post Kit Shop.
You can also change their slot one and slot two weapon parts. Yukulf uses the same slot one items as Golban, but the other slots for Yukulf and Hargna are the same as for Ryle, Golban, Runatie, and Karin.
Yukulf uses the same armor as Ryle, and Hargna uses the same armor as Rumil.