These kids deserve some love!
Romantic plotlines in shounen series most of the time feel awkward and unnecessary: usually they don't give characters' depth or any kind of substantial development. Quite often the romantic interest for the shounen protagonist just plays the role of a convenient damsel in distress; sometimes this person becomes a rival, or any sort of competitor, and that adds a bit of spice to the whole story.
The problem with romance in shounen stems from the fact that authors don't want to write interesting female characters and don't want to openly add same sex relationships.
Enter Tatsuki Fujimoto, who tackled this issue in Chainsaw Man and created a possibility of canonical pairing in a shounen story that people want to cheer for.
Asa and Denji, two very broken kids, are looking for validation in life, for love, for affection, but the world seems to disagree. And it works because Asa is as much fleshed out as Denji. She's compelling, important to the story, and extremely relatable — sometimes in the cringiest way, but still. Asa is a properly developed character, with her own battles, her own wishes, her own desires — she's not just a function that the author stuck into the story with the only purpose being a possible love interest as an afterthought.
Why Asa Isn't Your Typical Shounen Love Interest (And Why That's a Good Thing)
Majority of the shounen authors try to not write complex female characters because they believe that the proper emotional and intellectual intimacy is possible between two equal characters — hence the sometimes even unintentional yaoi bating that comes from the interactions of fully fleshed male characters.
The only way to break this cycle is to introduce equally developed female characters — and that's exactly what Fujimoto did with Asa. Not only her: Fujimoto is constantly introducing female characters that seem wonderfully multifaceted and layered, driven by many different things and not just one purpose. And it's not just Chainsaw Man: his one-shots that were published before also feature interesting girls.
Female characters in Chainsaw Man, not only Asa, are not created purely for fanservice and aren't there to simply fawn over the main character. And for now Asa seems like one of the healthiest and most realistic options for Denji.
We'll be rooting for them. But who knows how many plot twists await us.