I think Kate is H! She’s never been suspected of any wrongdoing yet she has lied about things that would jeopardise her credibility (which she knows). Plus she has the most storylines with the hinted UC with criminals, dying declaration, link to Akers, better hours but never at home, etc.
Jed Mercurio said he likes to write conflicted characters and ethical conflicts. Steve has been standing up to corruption since S1E1 and proved his integrity across all series. If Kate is H (she’s mega dodgy) it would be the biggest moral dilemma for him as she’s his best mate. It’s foreshadowing that 2 people reminded Hastings about not knowing Dot was bent!
I ‘DEFINATELY’ think H will be a main character!