Katida Lanco will rejoin during the second part, not before, if you:
1. Yell at her in the hospital when in Kashtano (spelling?)
2. Buy her real flowers
3. Side with Nova Nacio (when there is war between Nova Nacio and Kalymnos)
If you do not do that, she will not rejoin you at all and you end up fighting her. Hope it helps.