I was not feeling this one. At all.
Like the Adiron/Jade installment, the plot felt very "discover as you go." The main tension in Mathiras and Helen's relationship is her innocence, which sort of plays into an "age" issue depending on how one classifies maturity. So, this had to be addressed in text, and Ruby understood this very well.
Unfortunately, the way she went about "maturing" Helen was just weird and unsatisfying. Essentially, she went the route of "Helen has been modified genetically to be both extra pleasure-oriented AND physically adept at snapping necks. Therefore, because she is deadly and can save Mathiras, and because she is biologically adult, she is adult and there is no issue with her banging."
And while I personally am not one to grab the pitchforks the moment there's a questionable age gap or uneven power dynamics between fictional couples, I did think this conflict could have been solved in a wayyy less contrived manner. The entire plot with Helen going into sleeper cell mode and murdering indiscriminately was dumb and unsatisfying.
I would have vastly preferred a plot centered around Helen looking for details on her species, the qura'aki, and coming into her own during this journey with Mathiras at her side. It would have added a lot of cool lore to the universe while also focusing Helen's growth/readiness for sexual and romantic intimacy less on the fact of her physical state (genetics, physical age) and more on her emotional state (lived experience, mental resilience, self-reflection). Because the issue with Helen/Mathiras was always that of innocence and experience, not the age of her body, this would've been a more satisfying story, in my opinion.
The rest of my complaints are really about how damn bored I was throughout. The story took way too long and was too unfocused. Zebah and Bethiah were fun additions, but Mathiras felt off. His voice with Helen felt more like a father mentoring a child than a lover.