Yes, Minthara works for both good or evil characters and has more than enough affinity points by simply having her with you throughout ACT II or ACT III.
By the time I personally am done with ACT II she has so much affinity she immediately triggers a romance after the throne room discussion back at camp. Otherwise gain enough approval in ACT III and she'll initiate romance there. There's plenty of things she loves and a vast majority of those things do not involve being evil, in fact she hates unnecessary evil. For example merely speaking with the Thorm relatives in ACT II gives her massive affinity boosts.
