Severus Tobias Snape

Duality
For almost the entire series, he appears villainous: cold, caustic, and unfair to Harry and the Gryffindors. But behind the mask lies a deeply damaged man. His true role as a double agent is revealed only in the finale.
Love for Lily
This is the core of his personality. Having fallen in love with her as a child, he carried that feeling throughout his life. For her sake, he betrayed the Death Eaters, became a spy, and protected Harry, whom he couldn't stand—the son of James, his abuser, and also Lily's son.
Resentment and bitterness
As a child, he was bullied by James and Sirius. This left him withdrawn, cynical, and unforgiving. He takes these old grievances out on Harry, seeing James in him.
Intelligence and composure
A brilliant potion maker, Occlumens, and master manipulator. He knows how to conceal his emotions and calculate one step ahead. This makes him an ideal spy, but also cold and stern.
Courage and sacrifice
Despite his fear, he carries out a most dangerous mission. He kills Dumbledore at his request, knowing he will ruin his reputation forever. His final cry, "Always," is that love is stronger than death and hatred.
Cruelty as a defense
Being rude to students is not only a consequence of resentment, but also a way to keep a distance and not show weakness.

Severus Snape was born on January 9, 1960, to Tobias Snape, a Muggle, and Eileen Prince, a witch. His father was abusive and alcoholic, and his mother was downtrodden. His childhood in the poor neighborhood of Spinner's End was unhappy and shaped his withdrawn, embittered character.
As a child, he befriended Lily Evans, a neighboring witch, and fell in love with her. But at Hogwarts, their paths diverged: Lily was sorted into Gryffindor, and Snape into Slytherin. He became close to future Death Eaters, and James Potter and Sirius Black constantly humiliated him. In 1976, after a particularly brutal bullying, Lily stood up for him, but he angrily called her a "Mudblood." This ruined their friendship forever.
After school, Snape joined the Death Eaters. Hearing a prophecy about a child capable of defeating Voldemort, he relayed it to his master, unaware it might be Lily's son. When he realized this, he begged Dumbledore to save her. Lily died, and Snape swore to protect Harry. He became a double agent, spying for the Order of the Phoenix within Voldemort's camp.
From 1981, he taught Potions at Hogwarts, and from 1996, Defense Against the Dark Arts. He was the Head of Slytherin House. In 1997, at Dumbledore's request, he killed him to save Draco Malfoy and strengthen the Dark Lord's trust. He became headmaster of Hogwarts under Voldemort's regime, secretly protecting the students.
He died in 1998 in the Shrieking Shack, slain by the snake Nagini on the orders of Voldemort, who mistakenly believed Snape to be the master of the Elder Wand. Before his death, he passed on his memories to Harry, revealing his true role and love for Lily. His last words were: "Always."