After failing to save her own partner, the superhero called Nova is a haunted shell of her former self until a woman from an alternate reality appears seeking help – a woman who looks exactly like her dead partner.

How about this:

When her dead partners look-alike comes to her for help, a disgraced superhero gets a chance for redemption when the two hunt her sidekicks killer.

When the doppelganger of her dead partner arrives from a parallel universe desperately seeking her help, a mystical healer must overcome her own feeling of guilt if she is to regain her magical powers and save two universes from simultaneous destruction.

A lot of creative license here with the whole "save two universes from simultaneous destruction" bit -- but just wanted to up the anti...

I think the redemption angle does have to be 'so in-your-face'... To me it is the crux of the whole story (just going off your logline of course). Maybe it could be phrased differently..?

Nova is 'a shell of her former self'... why? Because she 'failed to save her own partner'. I'm assuming (maybe a bit of a leap...) but that because she blames herself over the death of her partner she can no longer perform her unique healing abilities..? The only way over this is through some kind of redemption, which she has an opportunity to do when the doppelganger of her dead partner appears 'desperately seeking her help' (thanks to dpg, there) -- I guess redemption is her need, helping her dead partners doppelganger is her desire, or, rediscovering her healing abilities to help the doppelganger of her dear partner is her desire...

...'She?s not consciously seeking redemption, she just feels compelled to help...' -- need is an unconscious compulsion -- desire is conscious.

Also, seeing as though she is some kind of mystical healer, 'Superhero' I think gives a misleading perception of what kind of film this is. I hear Superhero and I think "The Avengers/ Superman/ Spiderman/ The Hulk/ IronMan etc etc etc... Maybe a Superhero is what she is, but she sounds more 'magical/ mystical' to me.

I think providing more context of the world in which this takes place might help to define Nova -- does it take place in our universe/ galaxy? In our present, or in our future? Obviously tricky with such a unique world (hero with magical healing powers/ doppelgangers from alternate realities...), and you want to keep it concise as possible, but in the end I think it would be worth it.

Very original -- Good luck.