Okay so HB has some anxiety and self hatred and guilt and grief( a lot). Now i am going to say that her mannerisms are at least partially because of this.
I think she speaks slowly and clearly because she isn’t in control and the pain that's inside of her is drowning her and that because of that she can’t feel her body either( like it not real or her bones want to burst from her body- having secrets including hiding your sexuality can do this as it makes you think your life is wrong and you get angry and lose yourself).
Now I could also say that she has a speech impediment perhaps and that another reason.
Or even that she is afraid of being misunderstood and has to be perfect.
I might be really pushing my luck here but I think she moves her hands because she is angry and scared at all times and its the only thing she can control and perhaps this is the only reality she can live.
I move my hands and body in this way often for similar reasons and because i have taught myself to stay so small and suppress it all- every emotion I have comes out in this way so happy me is flappy and Softbroom. ( I could also say neurodiversity)
To conclude I am a nerd and I am broken and I love HB and I hope you like her to because she is very special and that sort of warmth and acceptance that a character can be guilty but also good and kind and mentally ill and that mental illness is acceptable if the trauma is shared and you take some ( almost all of the blame). Anger is okay. Get help. I love you all.