Hans Beckert
Hans Beckert is the child murderer M revolves around—both the film's villain and its protagonist. He's a stout man with a baby face and habit of whistling Edvard Grieg's "In the Hall of the Mountain King." Beckert blends into the crowd, wearing the same long coat and fedora as so many other men. We get little access to Beckert early in the movie, as he's mainly depicted as a shadowy figure abducting small children. We finally get close to him as he's pursued, and we come to know him as panicked mad man.
Elsie Beckmann
Little Elsie is the only victim of Beckert's that we meet. The film opens on her playing ball and singing a disturbing song about the killer, shortly before we meet him through her. She's a child just like all the others, and her murder seems to be the straw that breaks the camel's back. After her disappearance, the police, the underworld, and the masses all mobilize to find and destroy her killer.
Inspector Karl Lohmann
Police Inspector Karl Lohmann acts as the icon of the entire Weimer government in this parable. He approaches his job with pomp and circumstance, but is more or less useless when it comes to proper policing. Under his watch, nine children have been murdered and the police investigation into the crisis has been ground to a halt. His plan to search insane asylums and apartments to ultimately find evidence of the killer leads nowhere, and his police don't gain any intelligence on the killer by any other means than pure chance. When Lohman takes in Franz who he arrested at a burglary of the power plant, he's shocked to hear that the gang had been hunting and effectively captured the child murderer. Lohmann wins in the end, getting Franz to lead him to the killer's holding place, but less by fate or wiles than pure luck.
Der Schränker
Schränker is the foil to Inspector Lohmann, acting as the head of the city's underworld. He is the one who devises the plan to hunt the killer using a sprawling network of street people, and he is the one who acts as judge in Beckert's sham trial in the cellar of the old distillery. There are a few ways to read Schränker in this film. On one hand, he is a cunning yet compelling criminal whose effective means can't help but endear him to us, the audience. On the other hand, his power represents the shadowy, violent players who gain influence as the government loses efficacy and power. In this case, we see Schränker fill a power vacuum ushered in by the rage of the mob—not unlike the Nazi party would do just a few years after the film was made.
The Blind Vendor
The Blind Vendor lends M a bit of a grim fairy tale quality. It's a delicious irony that the only person who can recognize the killer—an unremarkable man who hides in plain sight—is someone who cannot see. The Blind Vendor does, however, recognize the sound of Beckert whistling "In the Hall of the Mountain King." The Blind Vendor is one of the many fanciful street people we meet in the movie, acting as a part of the cast that provides just about the only levity we can find in this picture.
Franz
Arguably the most colorful character in the film besides Hans Breckert himself, Franz is a capricious adrenaline junky with a strange moral sense. We initially meet him at the underworld roundtable where the gang devises its plan to catch the child murderer. Franz is skeptical of the plan, swearing that they should leave this investigation to the police. This foreshadows Franz's later cooperation with Inspector Lohmann, when he reveals to Lohmann both that the gang had broken into the power plant to capture the child murderer, and where the murderer was being held. Of course, it's not any sort of cold calculating that leads Franz to cooperate with the police, but the fear of prosecution after Lohmann hits him with a bunk accessory to murder charge.
Mrs. Breckmann
Elsie's mother is one of the few people we meet in the film who is not a criminal, a government official, or a member of the mad crowd. She's the soul of the picture. Early on, we identify with her as an increasingly worried mother whose child has yet to return home, worried sick amidst stories of a child murderer on the lam. At the end, she gives the film's chilling final lines, remarking that no punishment of Beckert will bring the dead children back and pleading (with no one in particular) to take better care of the little ones. To her, the cycle of violence does little good.
The City
As film scholar Tom Gunning explains in his book The Films of Fritz Lang, M lacks "an immediately identifiable protagonist who organizes the point of view of the film." He goes on to explain that the child murderer Hans Beckert is barely seen for the first half of the movie, the policeman Inspector Lohmann is missing from many of the movie's key scenes, and the underworld boss Der Schränker appears in the film even later than Beckert appears, even if he does hatch the plot that successfully captures the murderer.
Instead, we watch the entire city moving the plot of the film forward: angry mobs fiending for justice, the network of street people watching every corner to catch the killer, and the underworld and law enforcement organizations working in parallel to investigate the case.
The Defender
The man appointed to defend Hans Beckert in the kangaroo court taking place in the distillery stands out as the lone voice of reason in the whole affair, and basically the lone voice of reason in the entire film (save Mrs. Breckmann's remarks at the end). Lang uses this character to clear up some of the moral ambiguity, giving the viewer a clear sense of what injustices are in fact unfolding here.
The Innocent Man
The Innocent Man makes an error of judgment talking to a strange girl on the street amidst mass hysteria about a child murderer roaming free. His harassment, first by a burly man and soon by an entire mob, is supposed to show us just how little justice actually matters to people who simply want a man to answer for these atrocities sweeping the city.
