Who is DCS Patricia Carmichael in Line of Duty and could she be 'the fourth man'?

Since then we've seen more withering put-downs and insincere smiles than ever before, with Carmichael firmly underscoring her reputation as one of the show's best-ever villains – as explored in our ode to Carmichael.

And some fans have been wondering if she is even more villainous than she might appear. Could she, in fact, be the elusive H – or the 'fourth man' as they are now known.

Read on for everything you need to know about her and what her return to the show could mean for AC-12.

Who is DCS Patricia Carmichael?

Carmichael was introduced during the penultimate episode of series five as the boss of AC-3, one of the other anti-corruption units in the Central Police Force.

She was brought in to question Ted Hastings about his suspicious behaviour and quickly developed a reputation as something of a formidable figure, grilling the gaffer regarding the vast sums of money found in his room and his failure to disclose the fact he was in debt.

And as if that wasn't enough, Carmichael also levelled some very serious accusations at Ted – accusing him of blowing John Corbett's cover in an act of revenge and making it clear that she hoped to expose him as 'H'.

Thankfully, Ted was exonerated when Steve and Kate uncovered evidence that implicated his lawyer Gill Biggeloe – proving that she had been framing him the whole time.

But despite that discovery, Carmichael remained unconvinced and seemed determined to prove Ted's guilt, only backing down when it emerged that one of her own officers, Tranter, was bent – which greatly affected her ability to pass judgement on others.

Since her return to the show in episode five of the ongoing sixth season, Carmichael has once again proven to be a particularly prickly thorn in Ted's side.

Upon arriving and being made the Senior Investigating Officer (SIO) into Operation Lighthouse, she immediately shut down surveillance operations on Ryan Pilkington and Terry Boyle – which obviously had some very major repercussions.

And she went on to play a key role in the interrogation scene in series six which saw Jo Davidson come under intense scrutiny, clashing with Steve and Ted in the process by changing the subject whenever the topic of 'The Fourth Man' was brought up.

Could DCS Patricia Carmichael be 'H'?

The trailer for the Line of Duty series ix finale has strongly hinted that we could finally learn the identity of H – or 'the Fourth Man' as they have become known.

Naturally, given her status as a, shall we say, unpleasant character, Carmichael has emerged as one of the names fans reckon could be responsible for the ongoing corruption in the Central Police Force.

It's not difficult to see why she might be in the frame – her close relationship with the undoubtedly shady Chief Constable Philip Osborne is suspicious enough in and of itself, as is the clear animosity with which she views Ted and the rest of AC-12.

When added to her decision to immediately shut down all of AC-12's ongoing surveillance operations, and her apparent keenness to direct the subject away from 'The Fourth Man' during Jo's interview, there are more than a few clues that she might be bent.

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The questions is, is she taking these actions due to corruption, or is she simply a jobsworth who follows her superior's word to the letter?

It's possible that she could be conspiring with Osborne, but it's equally feasible that she could just be blindly following his orders in the hope of further promotion within the Central Police ranks.

Line of Duty concludes on Sunday at 9pm on BBC One. Take a look at the rest of our Drama coverage, or check out our TV guide to see what’s on TV this week.