This page explains how to recruit Irenes in Chrono Cross: The Radical Dreamers Edition.
Irenes is a mermaid from Marbule; she’s the sister of Zelbess.
How to Recruit
The first time you’ll encounter Irenes is at Marbule in Home World. After speaking to Toma the explorer and learning everything he has to say, you can sleep in the house by the entrance of Marbule. During the night, Irenes will briefly appear on the bridge to the north. The next day, Irenes will formally say “hi”.
Afterwards, you should visit the S.S. Zelbess. When you enter the captain’s cabin, you’ll bump into Fargo and Irenes having a short conversation. From here, continue until you defeat the Sage to obtain the key to the Dead Sea. Return to the main deck of the S.S. Zelbess and climb the mast, then use the lift to reach the Magical Dreamers ship.
Enter the door that’s right near you. Inside, Nikki will explain his plan to restore Marbule. Once everyone has left, Irenes will ask to join your party. If you refuse, speak to her again before she changes her mind.