Beware the Nice Ones: He's mostly a friendly guy, but like Matt, he has a handful of moments where people realize that ticking him off is a bad idea.
Big Eater: Frequently seen snacking on various foods. It's even lampshaded by Natalie and Matt's constant teasing and when Alesha is savvy enough to bring him a pastry to bribe him into doing some research for her.
Dead Partner: And how—his backstory includes a partner who was killed, Matt's the one who accidentally reminds him of this, eerily Foreshadowing his own fate, and DI Wes Leyton (not his partner, but clearly a good enough friend for both of these tropes to apply) is gunned down at the beginning of the episode "Hard Stop".
DecemberDecember Romance: In "Honor Bound", we learn that he had a relationship with a similarly aged fellow officer. Problem is, she was (and still is) married, and one of their trysts provides him with an alibi for a drug theft he's been implicated in.
Disappeared Dad: Much like his US counterpart, he has a rocky relationship with both of his daughters thanks to his alcoholism and his two failed marriages (which are themselves due to his drinking problem).
Expy: As a rather rough-around-the-edges but golden-hearted veteran police officer with a drinking problem in his past, several failed marriages and a rather rocky relationship with his kids, and a rather snarky attitude towards life and his job, he is essentially a British version of Lennie Briscoe.
Heroic BSoD: Breaks down after his partner Matt is killed. It really doesn't help that the whole thing happened right in front of him and that he's already been through this with another partner. Has another break down after Wes is killed too.
Kindhearted Cat Lover: Ronnie is seen caring for and commiserating with Matt's cat during "Survivor's Guilt", and walking off with it at the end (in a deleted scene).
I Let Gwen Stacy Die: It's implied that he feels somewhat responsible for his previous partner's death, telling Matt, "There was nothing I could do". Later, after Matt himself is killed, he expresses a similar sentiment while confiding in his AA group—"I couldn't get to him in time"—and giving his statement to the other detectives—"I should have been with him".
Like a Son to Me: Thinks this way about Matt, obvious even before the latter is killed and stated outright when he talks with the mother of his killer—"The police officer who died? His name was Matthew Devlin and he was like... MY son." Not surprising, considering how he screwed up with his actual children, thanks to his drinking problem.
The Nicknamer: He often called Matt "Matty", "Matty-boy", or "Sunshine". Fan were NOT pleased when he once referred to Sam by the last one.
Off the Wagon: Comes very close to being played straight—note the scene in Matt's now deserted apartment where he stares at the beer bottles for what seems like an eternity as well as the ending where there's a glass of whiskey in front of him—but is ultimately averted in "Survivor's Guilt" with the help of his new partner, Sam. Comes even closer in "Hard Stop", where he actually goes so far as to buy a bottle of vodka before coming to his senses and walking out of the store without it.
Papa Wolf: He's very protective of Matt, his much younger partner who is of similar age as his children, and Alesha, too. He's protective of anyone younger than him who he can sense needs a surrogate father figure.
In "Anonymous" his underhanded attempts made to correct mistakes made in the initial investigation end up making Matt look dishonest and incompetent and thoroughly humiliated in court. Matt pointedly refuses to sit with him in court and later blasts him for putting him in such a bad position.
In "Haunted", when asked how he and his then-partner missed crucial clues, he admits to his alcoholism—and his partner's amphetamine addiction. The man reads him the riot act, mentioning the numerous things he did to help him during his drinking days—to the detriment of his own personal life—and rebuffs Ronnie' s feeble excuses—"I was under oath"—and attempts to make amends—"I only eat with family and friends. And I lost my family covering your ass."
Jimmy Valentine tries to pull this in "Honor Bound" and cites the many times he covered for a drunk or hungover Ronnie. But the fact that he expected Ronnie to repay him by turning a blind eye to him being a murderous Dirty Cop completely eclipses his argument.
Abusive Parents: Strongly implied—his contempt for a child abuser—"I know people like you", his feelings about victimized children in general, his empathy with an abused kid—"I've been that kid, Ronnie", and with a young woman who refers to her father as a "bastard"—"Are we related then, do you think?", but neither the extent nor specific perpetrator is ever made clear.
Badass Adorable: He's gorgeous, amazingly polite... and in at least every other episode he runs down, disarms and disables a suspect, all while never cracking that well-mannered veneer.
Beware the Nice Ones: He's a pretty nice guy... until/unless nasty abusers, rapists etc. tick him off.
(As they interrogate a suspect)
Matt: Come on, man. Don't make me play "bad cop".
Ronnie: You wouldn't like him when he plays "bad cop".
By-the-Book Cop: To the point where he once arrested a friend for using cocaine—right in front of him.
Fair Cop: He's an extremely attractive cop. Numerous people have reacted to his good looks, ranging from come-ons and compliments to sarcastic put-downs—even the man suspected of being behind his murder (he isn't) refers to him as "Pretty Boy Cop".
Forgotten Fallen Friend: Despite his importance as a character and his relationship with Ronnie, Matt is never explicitly mentioned again after the episode dealing with his murder. Even when another of Ronnie's friends, DI Wes Layton, is shot and killed, there is still no mention of him and how similar the two events were for Ronnie. The closest reference we get is that Ronnie's grandson, born on the day Matt died, is named Matthew. Despite this clearly being done in his honor, it still fails to spark any discussion of him.
Innocent Blue Eyes: Has blue eyes the heroic and innocent, good-guy variety. His eyes are the last we see of him.
Kindhearted Cat Lover: Alesha is delighted to learn he has a cat in the episode "Deal", and Ronnie is seen caring for it (and commiserating with it) in "Survivor's Guilt".
I Let Gwen Stacy Die: It's implied that he feels somewhat responsible for his friend's suicide because he didn't protect him from the priest who was molesting him, nor realize that he was contemplating killing himself.
Mistaken for Racist: The attorney for his killer implies that he was a racist simply because he was a white police officer. Alesha tearfully and angrily denies this.
Nice Job Breaking It, Hero!: In "Honor Bound", becomes suspicious of the murky events in the episode's prologue and continues to investigate despite several people warning him not to. It exposes the actions of a Dirty Cop... who tries to implicate Ronnie as payback, resulting in the latter's humiliation when his alcoholic past is brought up, as well as his ex-lover's own humiliation when their adulterous affair is revealed as well. By episode's end, Ronnie is devastated at the loss of two people he clearly cared very much about and Matt is forced to apologize for his part in the whole thing.
Officer O'Hara: He reveals his heritage while trying to gain the trust of a young prostitute he's interrogating—very noticeably thickening his accent and pinpointing almost exactly where she grew up. In another episode he states that he was given a lot of grief about it during his rookie years—"they called me "Mick" for the first six months because my family are Irish."
Pretty Boy: Charming, youthful-looking, blue-eyed and blond cop with a face of an angel.
Raised Catholic: Mentions having lit a candle for a murder victim, "Once a Catholic...", but he's also clearly disillusioned with his faith—in the episode "Confession", he states that he hasn't been in a confessional in over 20 years, and the events of that episode offer a pretty strong indication of why this has happened.
Rape as Backstory: By proxy. In "Confession", we learn that two of his childhood friends were molested by their neighborhood priest, but it is left ambiguous as to whether Matt was a victim as well. He denies it throughout the episode, but at the end, admits that he's repressed things so much that he's genuinely uncertain as to what, if anything, happened to him.
Contrasting Replacement Character: Despite some obvious similarities with his predecessor—young, good-looking, absent (and possibly abusive) father, there were some equally obvious differences—Matt's temper was generally limited to certain issues, whereas Sam appeared to have a short fuse about everything. Matt also clearly cherished the surrogate father he had in Ronnie. Sam seemed to resent Ronnie's attempts at being this—despite generally getting along, the two simply did not have the rapport that Matt and Ronnie did.
Disappeared Dad: He had one and he is apparently unwillingly this to his own son, only getting to see him once a month.
Heroic BSoD: Suffers an emotional shut-down throughout "Tracks", starting when he's unable to save a young victim of the train crash.
But Not Too Black: He's mocked about his light skin by a dark-skinned suspect, who alternates between seeming jealous—insinuating that he's had an easier time because of it—or being contemptuous—calling him a "mongrel". He didn't take too kindly to it.
Half-Breed Discrimination: As cited above, his reaction when a suspect calls him a "mongrel" indicates that this isn't the first time he's dealt with comments like this.
Naïve Newcomer: Subverted. He's new to the Homicide Division, but he's been a detective for some time.
Iron Lady: Despite her obvious grief and anger, she waits until she's alone in her office with the blinds drawn to cry over Matt's death, and she refuses to break down despite the overwhelming evidence that her father is a killer.
Mama Bear: Is fiercely defensive of her subordinates, takes it badly when someone is criticizing or harming them (e.g. Alesha's assault), and them screwing up provokes a reaction much like that of a disappointed/angry parent.
Chekhov's Skill: His previous experience as a defense attorney saves his bacon when he's charged with "perverting the course of justice".
Determinator: His reluctance to give a case up or use any means necessary to win is legendary, and actually comes back to bite him in the ass in his last episode: he gets arrested for perverting the course of justice after some evidence that would have exonerated a murder suspect six years previously was found to have gone missing. The same episode says that his conviction rate is ninety three percent.
Office Romance: He has relationships with his female colleagues. And his final scene with Alesha (as well as his reaction to her rape) reveals that he has feelings for her as well, even though it never gets beyond attraction.
Put on a Bus: Although acquitted of charges of perverting the course of justice, he still feels ultimately responsible for the Miscarriage of Justice that occurred—an innocent man went to prison, while a guilty man went free and killed several more victims—and decides to resign from the CPS.
Tranquil Fury: His matter-of-fact demeanor when he declares that he'd like to see Alesha's rapist "locked in a room with his victims" does little to hide the fact that he'd like to be included.
What the Hell, Hero? His "win at all costs" method of handling a case has led him to do some borderline reprehensible things, all of which tend to have the same theme:
In "Paradise" he takes advantage of his friendship with another lawyer, jeopardizing the man's career, in order to garner necessary information. Upon finding out what he's done, the other lawyer gives him a blistering "The Reason You Suck" Speech and rebuffs James' attempts to make amends (one of the few times he was genuinely sorry for the problems that he'd caused), saying, "I don't want to be your collateral damage anymore."
In "Anonymous", he humiliates Matt Devlin in court over an error made during the investigation, accusing him of being both dishonest and incompetent (despite knowing full well that he's neither), discrediting not only his testimony, but possibly his tenure as a detective. When Alesha blasts him for this, he dismisses it as "collateral damage".
"Skeletons", he embarrasses an ex-lover over their past relationship and declares that her sole reason for testifying against him is to get back at him for dumping her. Alesha tells him off for this, asking if the woman's reputation was, of course, "collateral damage".
Break the Cutie: She's adorable, but gets drawn to hell: raped by her doctor, loses her friend/mentor before they get a chance to explore their feelings for each other, and another friend is killed saving her life before they can explore their feelings for each other. Unlike most examples of this trope, she appears to have come out of each tragedy stronger than before.
Character Development: She starts out as a Naïve Newcomer, but grows more and more confident and competent as time goes on—by Series 2 she handles the questioning of several witnesses, by Series 5, prosecutes several cases on her own, and the final mention of her at the beginning of Series 7 is that she's been transferred to a senior position in another division.
Composite Character: As a black woman, she is a combination of the first junior ADA from the original Law & Order, Paul Robinette, and his female successors, especially Claire Kincaid, who also had feelings for her mentor. The episode where she was raped was a remake of the original Law & Order episode in which Elizabeth Olivet was raped.
Everyone's Baby Sister: As seen in the episode "Alesha", everyone in the group is infuriated by what's happened to her, they go all out to bring her rapist to justice. James can barely stand to watch the video of her rape, while Matt can't bring himself to watch it at all.
Iron Lady DI Chandler: I'd like to cut off his dick with a rusty hacksaw and ram it down his throat!"
Heroic Sacrifice: When she tries to entrap a doctor who touched her inappropriately, she gets raped. She essentially jumped on a grenade, knowing something bad would happen, in order to get justice for all of Merrick's other victims, and knowing he would never pay for her. Indeed, when Matt asks why she went back after he'd already assaulted her, she admits that she wasn't expecting him to rape her, but states, "Someone had to stop him. At least now we can lock the bastard up."
Idiot Ball: In "Alesha", where she tries to entrap a doctor who touched her inappropriately and gets raped.
Put on a Bus: In the season 7 premiere, it's mentioned that she was transferred to a senior crown prosecutor position in another office. This narrowly averts a literal Bus Crash, as she would have been a passenger on the ill-fated train otherwise.
Twofer Token Minority: Is black and female. But even though she's the only non-white member of the main cast, it's hard to really call her a "token", given that London's incredible diversity is well represented among the supporting players.
Idiot Ball: Chats with an opposing attorney about the tactics Jake is likely to use, then acts shocked when (a) the woman undermines their strategy by preparing her witness, and (b) Jake and Henry read her the riot act for what she's done.
Chekhov's Skill: Like James, was a defense attorney for a while. Despite nearly two decades having passed since then, he does an excellent, if ultimately unsuccessful job of representing an old friend on a manslaughter charge.
Papa Wolf: Much like DI Chandler, is fiercely protective of his underlings, yet doesn't tolerate mistakes from them.