The Defenders marks a culmination of the original Marvel/Netflix team-up plan that was announced back in 2013, with the crossover series bringing together the four New York-based heroes from Daredevil, Jessica Jones, Luke Cage, and Iron Fist. Though the threat they faced, The Hand, may not have been as cosmic or grand as the army of aliens seen in The Avengers, it was a big enough problem that none of them could face it alone. Elektra's resurrection and involvement with The Hand has detrimentally clouded Matt's judgement by the time The Defenders begins, and his intentions for a greater good cannot be trusted when he begins to fall out of step with the rest of the group in his attempts to get through to Elektra.
Matt and Luke share a lot of similar qualities in that they are both selflessly fighting for the betterment of their neighborhoods; however, Luke is surrounded by characters who encourage his behavior whereas Matt's closest friends want him to lead just one life. Granted, Luke doesn't have an alter-ego or mask to hide behind like Daredevil, but that doesn't stop authorities from lumping him into the same vigilante category as the Devil or The Punisher.
In all of the series' that preceded The Defenders, it's easy to see Matt Murdock as the only one of the four to have accepted his role as a protector of the people. The Devil of Hell's Kitchen may be "retired" as The Defenders begins, but Matt was the first to embody the qualities typically showered upon heroes. All four of The Defenders main characters help in their own way, and while this team lacks a singular leader, it's obvious that Luke Cage is the group's voice of reason.
Luke Cage Doesn't Hide Behind a Mask
After Luke is released from Seagate, he and Claire barely have enough time to get that coffee they'd been waiting two years for before they're interrupted by Misty Knight. A lot has changed in Harlem since the showdown between Diamondback and Luke Cage, and even though Luke has been gone, he knows something is going on. Set on bringing down Mariah and Shades, Misty corrects Luke's trajectory to the more immediate concern of Harlem's youth turning up dead.
Instead of getting into police matters, Misty suggests Luke could help Harlem in other ways - like guiding its youth in a better direction. She pushes him to be more like Pop and help kids before they get into trouble, by showing them they don't need to turn to a life of crime to get a leg up in this world. Luke becoming a mentor in order to effect change is exactly the kind of start he's looking for; with his past finally behind him, he is free to move forward. He's still hesitant to accept the "hero" mantle, but also acknowledges that his biggest concern is being able to help people.
He understands that living openly in the community he wants to protect requires a different approach to what is going on in Hell's Kitchen. Pop's spirit weighs heavy on Luke in the early episodes of The Defenders. As he beings to peel back the layers of what is happening, he justifies his actions in the name of stepping in before other innocents are hurt because it's "what Pop would have wanted." It's not until he tries to get through to a young man, Cole, who has already lost his brother and sister to crime syndicates, that Luke decides he's all in with getting to the bottom of what's going on. Unlike Matt, Luke doesn't have the luxury of an alter-ego to do good deeds in the name of. His actions, for better or worse intentional or not, are always a reflection of himself and his community.