Didn't someone say Letty was raped by her uncle or something as a teen? If so, then this kid has a lot of issues. Sexually abused, clearly has an ED, has a creepy stalker ex, and it appears she rarely interacted with people outside of online/discord before the fishtank.
I'm wondering if the parents didn't teach her shit because they didn't want to put pressure on her due to sexual abuse/ED. It's not uncommon for parents to act this way, as if their child will break if they give them any discipline. Unfortunately, that kind of parenting doesn't do your kid any good long term, as we can clearly see with Letty here.