Initially, I thought Asa was wholly inferior to Denji as well, but her interactions with Denji so far are gold and certainly made her much more likable. I feel like if her character growth is written correctly the way Denji's character growth was, we could have the best Shounen female MC in ages.
Yoru is more interesting for now because of who she is, but Asa might catch up to her if she is not careful.