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I think that you have to see them as cause and effect. Yes, the writers forgot about her, so she ends up ignoring her kids. If a character kills someone, then they are still a killer. Because that's how they were written. An author's writing, or, in Thalassa's case, lack of writing, chooses the character's personality. As you said, characters are not autonomous people, therefore they are direct copies of how they are written. Thalassa hid the truth from her kids. It might have been bad writing, but it was still how it worked out in canon. At least that's how I see it! If I'm mistaken, though, please correct me. I only have my personal opinions on this as a basis, and I am always open to new ideas.
Going through the Apollo Justice Trilogy has made me such a Thalassa Gramarye apologist. Especially after seeing how she’s portrayed in fanfics.
It feels like so many people write her off as a negligent parent who’s hiding the truth from her kids on purpose. Guys, she was literally written out of the series by new writers who didn’t want to deal with the backstory AA4 set up. Characters are not autonomous people, they don’t get to choose what to do. Why blame the character instead of the writers who forgot about her in favour of their own ideas for a new game?