Every companion has three of their own unique companion quests that you must complete to earn this achievement. These do NOT count as side quests so you will not make progress towards the 20 required for that achievement. These companion quests are the following:
(You MUST complete all of Kurt's companion quests BEFORE completing The Prince's Secret)
Kurt:
- Missing in Action
- Amongst the Ghosts
- Settling Scores
Vasco:
- A Name for a Family
- Family Reunion
- Forever a Naut
Síora:
- Find Queen Bládnid
- Promises Set in Stone
- The Queen's Farewell
Petrus:
- The Vices of the Mother Cardinal
- Dangerous Bets
- The Sins of Father Petrus
Aphra:
- Islander Knowledge
- The Cave of Knowledge
- The Linking Ritual
Upon completing the final companion quest the achievement will pop.