If you think about it, we all know what Yuma’s father, Yugo, looks like and what happened to him, but the manga and anime never seems to mention Yuma’s mother. Whatever happened to her? Is she dead, did she and Yugo have a divorce, or what? Because there had to be a relationship with a woman if Yuma is Yugo’s son. (Or is Yuma adopted?)
As of current writing, Yuma's mother has never been mentioned ever in the story, and not even in any interviews/Q&A sessions. (Otherwise, information about her in Japanese should come as popular/surprising, but I found none and only the same questions in Japanese with fan theories about her being a Neighbor or not, etc.).
Thus, it's also impossible to answer any questions related to her: whatever happened to her, is she dead or divorced, etc.
There are some fan theories if Yuma was adopted, but that's just fan theories.