The hero: Nova (Pictured in my avatar)
Powers: Almost unlimited Super strength, Basic Invulnerability, able to form shields or project blasts of energy, eye beams, hypersonic flight, limited super speed.
Weaknesses: Nova's often likely to charge headlong into a fight even against the unknown, her nigh invulnerability giving her a brash and sometimes careless attitude. While bullets and tank shells bounce harmlessly off her, Chi energy or magical attacks can be much more effective on her, and she cannot take a sustained assault of that kind for long.
Summary: almost the classic flying brick inspired heavily by Supergirl and Captain Marvel (the Marvel one). Not quite as strong as the iconic superman, but could probably take some blows from him and walk away from it. No "Kryptonites" given the limits I did impose on her, but some things are more effective than others.
Who I think would win: It really depends on what kind of damage Riftwalker does with his weapons and powers, whether he's capable of harming Nova with void energy or tricking her into getting banished I think. He'd have to act and think fast though, and try not to get hit since Nova is -incredibly- physically strong.