Kirsten hasn't even been really mentioned to make an appearance in the main party yet:
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Based on what Cap. Sikorsky said in this article when Everlasting Saga was first announced (
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), it seemed like Kirsten and Painelle's issues were designed to have been resolved (Painelle's sister forgave Kirsten and wanted Painelle to forgive her as well), but this was also set years before the time of Iris's game, so she's changed a lot (arguably for the worse cuz the drugs have permanently made her lustful), so who knows if she has any real beef with Kirsten anymore.
Sikorsky also said Kirsten's mentality and reason for her actions was because she never had a chance to develop her mind properly since all she knew was war, which can lead her to act like a child at times when everyone least expects it. Either way, fun little mess but he laments that the ending was too short to give it a conclusion that eased people's minds.