#arget rambles

The first reason I thought about them as foils was because Dusa and Arachne both end up not pursuing romantic relationships with the god/goddess (Zag and Mel) in their lives. Beyond that, though, they share a lot of character traits:

They're both "monstrous" beings that are associated with something neither god nor mortal (Dusa is a gorgon who is decapitated, and Arachne from being turned into a spider.)

Both feel inferior in station, having no real role of authority in the world they occupy. Dusa is a servant, and Arachne is a woodland creature more or less without any other company.

Both perform skills based on their labor, Dusa through housekeeping and Arachne through weaving. These are both domestic skills practiced by women in the setting of Ancient Greece. Weaving in particular was one of the most important domestic activities for women of the house. Athena, Penelope, Arachne, the Fates - all the patrons and mythic weavers are, as you can see, women. Still, their efforts aren't really lauded as heroic or remarked on by other characters in the games, but in-narrative, both Dusa and Arachne are praised by Zag and Mel who see how useful/important/helpful their labors are.

Where these two characters differ dramatically is their position in the their society, their feelings towards that status, and their feelings towards themselves and, specifically, their bodies. Dusa is both dead and also an employee of the House whereas Arachne is living and is a freelancer weaver who lives alone in the woods.

From what I remember, Dusa isn't particularly overtly discomforted by her body. She makes jokes about the gorgons that spawned from her severed neck to Zag (who goes, "is that insensitive to ask about?", only for Dusa to brush his concern off). Her reticence to receiving affection from Zag is external to her body; it has to do with their roles, their statuses as prince and maid, and Nyx herself castigating her for overstepping and also overworking herself. She is a monster, but she is not treated as monstrous within the House.

I would also like to note that Dusa bears no real feelings towards the Olympians (who more or less aided her execution). Death has freed her from worrying over their influence in her life because... Well, she's dead.

Arachne, on the other hand, still lives. It's pretty clear from the get-go she has a love and hate relationship with her body. She's gotten used to many parts of her spider body and can even (somewhat sardonically) talk about enjoying flies or how threatening her fangs are, but she still compares her spider-hood unfavorably to the beautiful girl she once was. She also actively resents and hates Athena for the curse. Her body is as an ongoing punishment from the goddess, and she feels she cannot accept Melinöe's love due to her own self-loathing of her body. She rejects the romance not because external structures influence her, but because of her own internal state of her body.

We also see Arachne is also more confident in other ways— she talks back to Hecate directly when she discovers the Witch-Goddess' role in cursing her, whereas Dusa is a nervous wreck in front of all the gods, from Zagreus to Nyx. Arachne enjoys playing sinister tricks with all her beckoning into her web, and she is not beholden really to any master the way Dusa is. On the flip-side, Arachne feels far more isolated and lonely. She doesn't have a companionable relationship like Dusa does with Meg.

So you have these two "monstrous" women transformed by the will of the Olympians, and whose death or life has altered the perception of the key trauma in their life. You can see different ways they take up space in a social structure primarily made of gods and mortals. I think this directly connects both to the meta-perceptions of these characters as well as the themes of these two games.

I would posit that, in audience reception, Dusa, who is sweet and biddable and doesn't really talk about her trauma and arguably a victim through no action of her own, is generally beloved whereas Arachne, still resentful and openly hostile to her wrongdoers, is far less celebrated and held up as a mascot. I think it's not a reach to say that Arachne discomforts people and forces them to contend with imperfect victims and egregious punishment. I think this is epitomized in the (extremely uncomfortable) conversation Melinöe has with Athena which frames Arachne's plight as "did her arrogance/actions merit punishment?".