I feel like little bits of a bunch of the earlier comments are all correct: I think she threw him down the hill to "kill him". He didn't die, but the idea of him in her head did, so she has carried on raising him ever since, as like a separate, dissociated parent? Does that make sense? It explains a bunch of her behavior around him(though I haven't read this series from the start in years); she's just been raising this random kid for years that she has no connection/feelings for, because her "real" son is "dead". It's like there's Free, non-parent Seiko, and Over-protective mom Seiko but they're both the same person and in varying quantities. And then all of this could be absolutely wrong next chapter, sooooo