In the first chapter, the Hero (?) and the Demon Queen (?) are shown fighting each other using the MC's weapon. Probably this dark elf girl is gonna be the catalyst that will make the demons establish an amicable relationship with the MC and convince him to forge a sword for the Demon Queen.
What a terribly written and lazy introduction. There is literally nothing redeemable about her at the point that they just wave off that she threatened to kill them all.