Whether a deity is, "good" or "evil" varies amongst people. The reason for this being morality is not the same for everyone. I do not see her as evil by any means.
If you are looking for a deity to help with battle, strength, "war" (I use the quotations because many of us fight our inner demons and personal wars blah blah blah, metaphor.) Anyways, if you are looking for a goddess specifically in the Greek parthenon I would suggest Athena or Enyo. I prefer Enyo but that boils down to my personal bias.
Lastly, when it comes to, "summoning" many think it comes off a bit...rude to say the least. But reguardless of which side of the fence you are on with that word it is good to recognize that summoning is not always necessary or wise. Deities can choose to help you for nothing, they can choose to help you with a simple offering and prayer. But asking a deity for help when you know very little about them seems much like asking a stranger for a favor in my opinion, incredibly rude and probably not going to happen.
Information on Styx:
http://www.theoi.com/Khthonios/PotamosStyx.html
Information on Athena:
http://www.theoi.com/Olympios/Athena.html
Information on Enyo:
http://www.theoi.com/Daimon/Enyo.html
Invoke vs Evoke:
http://unenlightenedenglish.com/2009/06/invoke-vs-evoke/
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