Credits: Pegasus Knight, TearRing Saga Official Complete Guide
To successfully recruit Leteena, you need to follow these instructions.
Map 2 or 12
When at Verge Manor, pick Ezekiel to join your party. Note that it’s possible to recruit Leteena without him, he just makes it easier.
Map 8 (Reunion 1)
Send Kreiss and Arkis to Holmes’s group. Give Kreiss a Healing Drop and an Amulet if you have them.
Map 10 (End) to Map 13 (Start)
Visit Verge with both Kreiss and Arkis present and an event should trigger.
Map 13 (End) to Map 15 (Start)
With the Healing Drop in Kreiss’s possession, enter Verge and an event should trigger.
Map 19 (End) to Map 23 (End)
Before Reunion 2 occurs, visit Verge yet again with Kreiss and another event should trigger.
Make Kreiss defeat Marco, the leader of the Condor Squad, to find out where Leteena is located. Then visit the village where Rebecca would normally reside. If done successfully, Leteena should join instead of Rebecca.
Map 30 (Reunion 3)
If both Kreiss and Leteena are alive, an important event will occur and you will be forced to make a choice. If you pick the first option “No, it’s not possible”, Leteena will leave. If you pick the second option “I understand, if Leteena says so”, both Kreiss and Leteena will leave. However, if Ezekiel is present while choosing the second option, neither Kreiss nor Leteena will leave.
Alternatively, if you don’t have Ezekiel in your group, you could simply kill off Kreiss or Leteena before Map 26 ends.