In my playthrough, I killed her to save a grove. Now I want to try the non-lethal attack option and add Minthara to the team in Moonrise. Playing as a neutral character, will you be able to reach approval points high enough to make her want to open up to Tav? Is it only bad gameplay that gives you a chance to be friends with Minthara?
As long as you prioritize her objectives and principles above your own and exhibit decisiveness in conflict resolution you should be good to go as long as you also avoid displaying cowardice and as expected indecisiveness in conflict resolutions.
Basically she likes one that displays leadership qualities.
There are plenty of moments where MInthara will disapprove even when you go all out evil without any good reason or ambition behind it, hence why I don't look at her as one and doesn't guarantee high approvals necessarily, granted there are a few evil choices she does approve of within good reasons of course.
In my first blind playthrough with MInthara as companion was shocked to see just how high of an approval rate I got with her before even leaving Act 2, mind you I did not go the non-lethal route but had plenty of mingle with her in Act 1 so that could also have had a weight to it for me.
Hope this helsp you give an idea of Minthara, would love to hear what your favorite character is.