Here is the full list of all 9 Questr achievements.
Successful bribe your friend to stay.
Complete an entire Tier with the same friend.
Reach Tier 2
Complete a quest with over 100% Morale
Made up with your ex-friend.
Complete 3 Tiers with the same friend.
Reach Tier 4
Have more than 5,000 Gold at one time.
Successfully complete a Revenge Quest.