DS Steve Arnott

DS Steve Arnott is a firearms officer whose refusal to falsify a report after a botched anti-terror raid gets him sidelined into anti-corruption work he never wanted. Rigid, principled and quietly ambitious, he arrives at AC-12 resentful of the transfer, determined to prove himself, and instinctively suspicious of the colleagues he's meant to investigate.

Compston plays him as a man whose integrity is almost a liability, all clenched jaw and barely-suppressed frustration at a job built on distrust. The writing keeps testing whether his rulebook rigidity is heroism or self-righteousness, and that unresolved tension, played completely straight, makes him the show's moral anchor without ever making him dull.