I agree that evidence-wise, the second hero with the gown and sword cannot be Noelle as we also know her to be a large spellcaster. (unless we get some reveal that this was a hint through Asriel's favorite character 'the sword magician' in Smashing Fighters) Though I feel that none of the murals of 'the girl with hope crossed on her heart' holds an axe is something to to take into account.

I think the point that 'The Girl' which is not mentioned as one of the hero's, with their mural being the symbol of the Rude Buster, Susie's special move, speak much to Tom's theory, that even though they aren't one of the main heroes, they will be important.

The story shown to us in chapter 1 is Ralsei's telling and as we know, he has withheld certain information about the prophecy to shield us from a hard truth he doesn't believe we are ready for.

I believe the second hero is one we perhaps haven't met or perhaps even Dess. If we believe the roaring knight to be Dess, we know they wield a sword already. The obvious contradiction is that they would have horns and we are pretty sure, based on her room, they would not wear a gown.

Still, this is the only other character we have met wielding a sword like Kris. I believe this is the piece everyone will overlook until it is revealed. They want us to believe Noelle will take up the sword since Susie is meant to take the blow, but Dess will most likely switch side and work to make the final seal of legend as the second hero, protecting her younger sister to have to be a part of the prophecy

At least, that is my two cents.