Luwin is the Maester of Winterfell and a loyal servant to House Stark. He is a surrogate parent to Bran Stark and Rickon Stark in the absence of their family.
Luwin is a maester, a learned man of science and medicine, and serves House Stark of Winterfell. House Stark are Lords Paramount of the North. The North is one of the constituent regions of the Seven Kingdoms and House Stark is one of the Great Houses of the realm. Luwin is very loyal to the Starks and they value his wise counsel and experience. He delivered all five of Catelyn's children. He serves as a tutor to the male children.
Luwin organizes the preparations for a royal visit with Catelyn Stark. Catelyn spots Bran climbing Winterfell's walls and scolds her son in front of Luwin. Luwin stands behind the Starks as King Robert arrives. The next night Luwin delivers a letter from Lysa Arryn to Catelyn and Eddard Stark. He discusses its content with them and counsels that given Lysa's claims that the Lannisters killed Jon Arryn it is important that Eddard take up Robert's offer to make him Hand of the King. Bran is injured in what seems to have been a climbing accident.
Luwin tends to Bran's injuries and predicts that he will survive. Eddard goes to King's Landing to serve as hand and takes many of his retainers with him. Luwin goes to Catelyn to discuss finances and appointing replacements. Catelyn is distraught over Bran's condition and refuses to participate. Robb Stark agrees to work with Luwin instead. An assassin attempts to kill Bran but is foiled by Bran's direwolf Summer. Luwin meets with Catelyn, Robb, Theon Greyjoy, and Rodrik Cassel in the Gods Wood of Winterfell to discuss the circumstances of Bran's injury. Catelyn reveals that she found a blond hair inside the tower where he was found. Luwin cautions restraint and Catelyn plans to go south personally to deliver the news to Eddard. Bran awakens after she leaves but is paralyzed from the waist down.
Luwin meets with Tyrion Lannister in the main hall of Winterfell alongside Robb. They offer a cool reception until Tyrion provides plans for a saddle that would allow Bran to ride. Luwin tutors Bran on the regions and families in the Seven Kingdoms and notes that amongst House Greyjoy's noted skills is a tendency towards failed rebellions. Bran is upset at the continued absence of his parents and sullenly tries to provoke Luwin.
Luwin warns off Theon Greyjoy from Osha, a wildling prisoner captured near the castle. He then questions Osha himself. She claims that the White Walkers are on the move. He dismisses this as a fable, but Osha tells him that the Walkers were not wiped out thousands of years ago, but have merely been asleep. She claims that they are not asleep any more.
Eddard is arrested for treason after the death of Robert. A letter from Sansa reaches Winterfell, pleading with a disbelieving Robb to bend the knee to Joffrey Baratheon. Maester Luwin correctly identifies the letter as the work of Queen Cersei. Robb refuses to consider the request and sends out ravens, summoning Eddard's lords bannermen to assemble their armies at Winterfell. Robb marches south leaving Luwin to care for Bran. Eddard is executed for treason on King Joffrey's orders. Luwin tells Bran and Rickon the news of Eddard's death.
As Maester at Winterfell he becomes ever more of a father figure to Bran and Rickon with their father dead and their mother away. He provides council to Bran as he acts as Lord of Winterfell. Bran questions Luwin about his strange dreams and Luwin reveals to him the valyrian steel link in his Maester chain, the sign that he studied the "higher mysteries" - magic - and asserts that if magic ever existed it has left their world long ago.
Theon Greyjoy lures the garrison of Winterfell away by feinting at Torrhen's Square and then seizes the poorly defended castle. Luwin is able to send word to King Robb Stark before being captured. Theon has Bran publicly surrender the castle to him and then executes the defiant Ser Rodrik Cassel. Osha frees the boys and flees the castle.
Theon has Luwin accompany him as he attempts to hunt the escapees down. Luwin begs him to spare them. They lose their scent at a nearby farm. Theon has Luwin sent home and then later returns to Winterfell with the corpses of two children, publicly claiming that they are Bran and Rickon to Luwin's horror. Days later Luwin spots Osha sneaking into the catacombs. He realizes that the corpses were farmhands and follows her to confirm his suspicion. She tells him they doubled back through a stream to mask their scent and he urges her to keep the truth of Theon's actions from the boys, saying that they have suffered enough.
Winterfell is surrounded by a northern army under the command of Ramsay Snow, Lord Roose Bolton's bastard son. Luwin tells Theon that holding the castle with only twenty men is impossible and suggests that Theon join the Night's Watch, gaining a chance to redeem himself. Theon refuses and tries to rally his men for battle, but they betray him; Dagmer knocks Theon out, with the intention of handing him over to the Northmen in exchange for their lives. When Luwin tries to intervene, Dagmer stabs him with a spear.
The fugitives emerge from their hiding place in Winterfell's crypts and find Winterfell sacked and Luwin dying in the godswood. He insists that the boys go north to the Wall as there are too many enemies in the south. He tells them that Jon will be able to keep them safe and tell their mother of their survival. Luwin affirms his pride at having served the Starks, particularly in raising their children. He sends them away and asks Osha to give him a quick death.