Sirius Black

Sirius Black in his school years
Sirius Black was a Gryffindor, one of the Marauders. Already at Hogwarts, he contrasted starkly with his family's ideals: he was sorted not into Slytherin (where all Blacks traditionally went), but into Gryffindor—thus immediately demonstrating that he did not share his family's beliefs about the superiority of pureblood wizards.

At school, Sirius was bold, free-spirited, and rebellious: he loved pranks, clung tightly to his friends, and had no patience for injustice. Along with James Potter, Remus Lupin, and Peter Pettigrew, he studied Animagus and became an Animagus (transforming into a huge black dog—hence the nickname "Padfoot"). They did this primarily to be by Remus's side during his werewolf transformations. Another of their shared endeavors was the creation of the Marauder's Map.

The Black family (in the context of Sirius's school years)
His parents, Orion and Walburga Black, were strict and demanding adherents of the "pure-blood" tradition. They expected their son to follow family traditions and were disappointed in Sirius due to his views and his sorting into Gryffindor. The atmosphere in the house was one of reverence for "blood purity" and disdain for those the family considered "blood traitors."
The younger brother is Regulus Black. During his school years, Regulus tried to live up to his family's expectations, being more obedient and "proper" in the Black family's eyes; the brothers were not close, and their values ​​diverged.
Relatives. It was customary in the Black family to erase from the family tree those who did not live up to the family ideals (by burning their names on a tapestry). Sirius felt like an outsider in this house from an early age and became increasingly distant from his family.
Uncle Alphard Black. Of all the relatives, he was Sirius's favorite and later helped him, for which he was also removed from the family tapestry.

Marauders
James Potter (Prongs) is a Gryffindor, a charismatic ringleader, an excellent Quidditch player, who loved to joke around with others, but without malice.
Sirius Black (Padfoot) is a Gryffindor, a rebel who consciously rejected the values ​​of his family (Slytherin, blood purity), and the most daring of the four.
Remus Lupin (Moony) was a Gryffindor, intelligent and reserved, who tried to balance the group; he hid the fact that he was a werewolf, and his friends helped him during his transformations.
Peter Pettigrew (Wormtail) was a Gryffindor who, in the shadow of others, was drawn to strong personalities and participated in the creation of the Marauder's Map.
Other important figures of the era
Lily Evans was a Gryffindor, a talented witch, who did not accept James's arrogance and was friends with Mary McDonald.
Severus Snape, a Slytherin gifted in potions and the Dark Arts, became increasingly withdrawn due to bullying by the Marauders; he was friends with Lily.
Marlene McKinnon is a Gryffindor, feisty and outspoken, one of the Marauders' brightest peers.
Dorcas Meadows is a capable sorceress (who would later become a prominent member of the Order of the Phoenix in canon) who stood out for her independence and courage at school.
Regulus Black is a Slytherin, Sirius's younger brother; during his school years, he followed his family's guidelines, but was already showing his own doubts.

The Marauder Era is the period in the Harry Potter universe during the Hogwarts years of James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew (roughly 1971–1978).

Who are the Marauders?
These four Gryffindor friends were renowned for their mischievous pranks, strong friendship, and important inventions. Each of them had their own nickname, which was used on the Marauder's Map:

James Potter - Prongs;
Sirius Black - Padfoot;
Remus Lupine - Moony (Moony);
Peter Pettigrew - Wormtail.
What is this era known for?
The Marauder's Map. Their most famous creation: a magical artifact that maps every corridor of Hogwarts and the location of every person in the castle. It later came into the possession of Fred and George Weasley, and then Harry Potter.
Animagus. Three of the four (James, Sirius, and Peter) became Animagi to support Remus during his werewolf transformations.
Confronting Voldemort. After graduating, the Marauders joined the Order of the Phoenix and fought the Dark Lord and his minions.
Complex relationships with other characters. For example, the Marauders had a tense relationship with Severus Snape—this dynamic significantly impacts the perception of their era.