I encoundered a bug where Nel attempts to change equipments after every fight. Also Nel got Celica killed during that session.
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For what i know Nel always changes your gear for what she considers the best before battling, you can dissable this by talking with her about "advices". And yeah there is a chance nel can get you killed if you allow her to go to places when you can get constantly grabbed/paralyzed like the beach (with no way of stopping paralysis) or the desert if you are sent alone.