In addition to good affinity, you also need to earn the endings for Reimi, Meracle and Sarah (in the same playthrough) AND have these specific PA's triggered:
Again, refer to "Reimi's Ending" - all these are listed in order they appear during the game in the walkthrough.
- On the fifth leg of the flight to Cardianon, speak to Reimi in the room with the battle simulator.
- After recruiting Myuria but BEFORE completing the Purgatorium, return to the Calnas to speak with Myuria and Bacchus in the crew quarters.
- Either during the flight to En II or while docked there, trigger an event with Arumat and Reimi. Make sure to answer that Crowe is either your best friend or your enemy.
- Place a funeral boquet near the remains of Crowe's ship in the Halls of Termination on Nox Obscurus.