To casual viewers of the Marvel Cinematic Universe, Ronan the Accuser is little more than "that angry blue alien with the powerful mallet." But Marvel Comics fans know there's a lot more to the infamous enforcer of Kree law. Ronan has a rich comics history that stretches back to the late '60s, and he is named among the best Avengers villains. He's one of the most significant members of the Kree, a humanoid, blue-skinned race that originates from the planet Hala. With their fondness for conflict and technological innovation, the Kree are basically Marvel's Space Romans.
Ronan is also a member of the Kree Public Accuser Corps, a heavily militarized police force. As the Supreme Accuser, Ronan is effectively the Attorney General of the entire Empire, answering only to the Imperial Minister and the Supreme Intelligence - the bodiless, all-knowing being that governs the Kree. As referenced in Captain Marvel, Ronan is a long-time antagonist of Captain Mar-Vell - Carol Danvers' male precursor in the comics. But his undying devotion to his people has also put him in the crosshairs of the Fantastic Four, the Guardians of the Galaxy, the Inhumans' Black Bolt, and even the Supreme Intelligence itself, making him one of the most notable Marvel villains ever written into being.
During Marvel Comics' three-part Maximum Security event in 2000, Earth is designated as a prison planet by the Intergalactic Council as punishment for its inhabitants causing so many problems throughout the universe. In fact, as Ronan explains to an argumentative Warbird, the entire sector is "quarantined" to contain Earth's inhabitants.
The planet-sized facility holds lawbreakers as big and powerful as Ego the Living Planet. Ronan later reveals his role as warden is essentially community service.
Ronan has always been fiercely loyal to his people, but when it comes to the Supreme Intelligence - the hivemind being that rules over the Kree - his allegiance often falters. Their tumultuous history began in 1969's Captain Marvel #16 where Ronan's feelings of unease toward the mysterious entity turn mutinous. Along with Zarek, the Kree Imperial Minister, he attempts to dismantle the Supreme Intelligence, a plot that's foiled by the first Captain Marvel, Mar-Vell.
Mar-Vell gets a snazzy new outfit and some cool new powers as a reward, while Ronan gets sent to space jail.
Following the drama of Secret Invasion - in which the shape-shifting Skrull race infiltrate Earth - the Inhumans seek to reconnect with their Kree creators. Black Bolt becomes the Kree Emperor, and after suffering a heavy loss at the hands of the former Inhuman king, Ronan submits.
In order to cement their new relationship, Ronan demands he take Black Bolt's sister-in-law, Crystal, as his wife. While Crystal is initially all-business about their marriage, her sympathy and warmth towards the Kree people endear her both to them and her new husband (who is already pretty smitten). When Black Bolt eventually steps down as Kree Emperor, he dissolves Crystal and Ronan's marriage, much to the pair's mutual dismay.
While he may come across as a bit rough around the edges, Ronan's blue skin is a badge of honor in Kree society, distinguishing him as someone of noble birth (and sparking a lot of racial tension on Hala). As a kid, Ronan enjoyed all the privileges most in the upper classes do, like a good education and a full ride into a coveted profession.
For the Kree, that means being trained in the Kree Public Accuser Corps, i.e. the Kree's judge, jury, and executioner department. While he excelled in his job and climbed the career ladder quickly, Ronan's superiority complex gets him into hot water later on when he refuses to bend the knee to the Supreme Intelligence.