Head cannon that Rio can go to the afterlives of the souls she's collected. And most of the time, she doesn't. Too many names, Too many faces, too many people to rember. Should she start to care, it would be too much for her. That is, until she meets Agatha. The first time she sees her, Agatha is standing over a dead coven- and Rio feels herself grow curious. As the bodies pile up, and Rio encounters her over and over, she slowly falls in love, delaying taking souls to their final destination for as long as she can, trailing Agatha. When the finally meet, face to face, Agatha tells her that she knew something was trailing her! in a tone that is so Agatha Rio can't even deny it any longer, she is in love. When Nicky is born, Agatha pleads with her, and she knows that she's going to break Agatha's heart one day.
And then that day comes. And Agatha mourns.
And Rio does something she's never done before; she goes and greets a soul. Nicky's soul. And they bond and laugh together, companions in the afterlife, and Rio makes a point to visit as often as she can. And some centuries she gets busy, in times of great plague and great wars. But she never goes more than a year or two without a visit to Nicky. She Brings news of his mother, when he asks, telling him all about the country's she's seen, and the life she's living. She makes a point to tell him how proud his mother is of him. She tells him some of the truth, saying that his mother spread their song so far and wide that every witch on the globe knows it. She leaves out the part about his mother committing large amounts of murder. She tells him about how his mother thinks of him, all the time. How she still wears a pendant with his hair in it, and she wears it everyday.
Had Agatha gone to the afterlife, Nicky and Rio would be waiting at the edge, to greet her into rest. Of course, she didn't do that. She went back to the living. Rio Knows that Agatha knows how much she hates ghosts. They insult her. If Rio wasn't so sure in Agatha's avoidance of her son, she might think it was just to spite her. She tells Nicky that, anyways. Tells him that Agatha didn't want to come with her to the afterlife because of how betrayed his mother felt when Rio took him. That only makes Nicky angry, though. Because He knows Rio, Knows her like she's his other mother, has forgiven and understood Rio for ages now! Why can't his mother forgive? Why doesn't she want to see him?
And through tears, Rio tells him that he would never understand, because being a mother without her child is like a gaping wound that never heals. Because Being a mother was Agatha's favorite job in life, and she's had many. Because if someone took Nicky from Rio, She would grieve for the rest of eternity. She would never forgive.