(or open) is the mother of Art's child, whom he cares very deeply for.
Like I mentioned in the ad for Art's daughter above, Sonia and Art were casually dating when Sonia got pregnant. Art was shocked into silence by the news, and walked away before they could really discuss anything. Sonia decided (for reasons that are completely up to you) to keep the baby, and let Art know that she didn't expect anything from him going forward. Despite his earlier reaction, Art ultimately chose to be a part of his child's life.
All of this is discussed in more detail in Art's app, which I recommend at least skimming through for more information on their life together. The gist of it is this: in preparation for the baby, Sonia took a year off from college and moved in together with Art. After the baby was born, she and Art maintained a platonic relationship (with the exception of one night that they never talk about, for their own respective reasons) and managed to raise a healthy, well-adjusted girl they're both exceedingly proud of.
Things like her job and her history before she met Art are open, but I'd like her to be passionate about helping people, as that (at least in part) was what attracted Art to her in the first place. I also imagine that she's got at least a decent relationship with her parents, as I like to think that they helped out quite a bit while Sonia was going to school and raising a baby at the same time.
Her current relationship status is also open. She was the one who suggested to Art that they just be friends after she found out she was pregnant, and then to find separate places when they were planning to move to San Diego when their daughter was four (which, by the way, was also prompted by her job transferring her). She and Art talked about getting married quite a bit, but always decided against it. The one time they slept together while their daughter was growing up, Art nearly proposed, which actually sort of freaked him out. That's why he never talks about that night; Sonia doesn't, either, and the reason for that is, again, completely up to you. Maybe she's found "the one" and is now either engaged or married - that'd actually be super interesting to explore.
So yeah, Sonia and Art's relationship is pretty complicated, and I don't know if any of my rambling in this ad made any sense. If you have questions, please feel free to shoot me a PM or add me on Discord! I just think it'd be a lot of fun to play out her dynamic with Art as co-parents and friends and whatever else because they mean so much to each other and their lives are always going to be intertwined because of their daughter and yeah. Please take her.
Her spirit is wide open. If you want someone from the same movie as Art's spirit, Annie Hughes from The Iron Giant (who's also a mother!) might be a good choice. Her suggested face claim is Eva Mendes, but I'm very, very open to suggestions - I'd absolutely love it if she was Latina, but it's not a requirement.