Hi There Suppersom! Thanks for starting this thread! I'm happy to answering questions about the game if you are running into any difficulties with strategy/progression.

Recruiting Katnir

After completing the forest task, Katnir will offer to lower his fee to $1000. If you don't have enough money or refuse to pay, you will be in debt to the forest. This must be paid before he can join. When you have $1000+ Wandering around the forest town will give you the option to pay your debt. After your debt is paid, randomly encountering Katnir through dueling and beating him will give you the option to recruit. It's a level 4 encounter so I suggest saving at the start of battle and using two air units + Ruwin if possible. Good Luck!