Aemon is the maester at Castle Black and is one of Lord Commander Jeor Mormont's closest advisers in the Night's Watch.
He was born Aemon Targaryen and is the last Targaryen in Westeros. He is the great-uncle of Daenerys Targaryen, currently in exile in Essos and unaware of his existence. Aemon's origins have long been forgotten by most, as he has remained dedicated to his vows as a Maester and a brother of the Night's Watch for decades uncounted.
Aemon was born into the royal House Targaryen, the son of King Maekar I. Originally known as Prince Aemon Targaryen, he was not his father's eldest son, so he eventually joined the order of Maesters. Aemon gave up all rights to inheritance when he took his vows, which also included dropping his surname "Targaryen".
Aemon's older brothers later died, and despite being a sworn maester, there were those who felt that the crown should pass to him. Aemon refused the throne, however, and it passed to his younger brother, who became King Aegon V. Aemon then joined the Night's Watch to remove himself from the political intrigues of the royal court, and those who wished to set him up as a rival against his brother.
Many years later the crown passed to Aegon V's son, Aemon's nephew Aerys II, called "The Mad King". Aerys's growing insanity ultimately led to the outbreak of Robert's Rebellion, which saw House Targaryen deposed and practically exterminated. Upon hearing about the brutal death of his family in King's Landing during the rebellion, Aemon bore great sadness which has never left him.
Around one hundred years old, Aemon is thought to be one of the oldest living men in Westeros, though he has retained all of his wits and his mind is still very sharp. He has served as the Maester at Castle Black for many decades.
Aemon has long since gone blind due to old age. It is not clear when he went blind (he was not born blind), though he has stated that he was already old and blind during Robert's Rebellion. As a man of learning, he laments not being able to read anymore, even though Castle Black ironically has an ancient and well-stocked library consisting of thousands of books. Because one of a maester's basic duties is to handle messages sent to and from a castle by messenger-ravens, Aemon has to rely on the assistance of others to physically read the letters for him.
After Tyrion Lannister has arrived at Castle Black, Maester Aemon asks him to urge the Queen to send more men to the Night's Watch: there is a severe lack of men to defend the Wall. They discuss the coming winter, and Aemon assures Tyrion that the next winter will be long and terrible. Lord Commander Mormont is worried about rumors of White Walkers abroad north of the Wall . He sends Benjen Stark ranging to investigate.
When Jon Snow and the other new recruits are done with their first training, Maester Aemon gives the new stewards their assignments. Jon is to become Jeor Mormont's steward, which is a great disappointment to Jon: he had hoped to become a ranger. Samwell Tarly is assigned as Aemon's steward, to help physically read letters delivered by messenger-ravens to the blind old man. Aemon chides Jon that he swore a vow to serve the Watch in any capacity, then departs. Sam then reminds Jon that being a steward for the Lord Commander might mean that Jon is being groomed for a high command position in the Watch. The bodies of two of the rangers who accompanied Benjen are found near the Godswood. The Lord Commander plans to have Aemon examine them, but they return as wights and have to be burned before he does so.
When Jon hears about Eddard Stark's imprisonment, he is torn between his loyalty to the Night's Watch and his love for his family. Maester Aemon intercepts Jon as he is preparing to desert the Watch. Aemon says he is aware of Jon's struggle and reveals to him that his real name is Aemon Targaryen, and that he is a long forgotten uncle of the Mad King. He sat and waited on the Wall whilst he heard about his family being butchered during Robert's Rebellion and the few survivors fleeing into exile, and did nothing: after having joined the Watch, his duty was with his brothers there and no longer with his House. Aemon explains that this was a difficult decision, yet he had to make it and that one must live with the consequences of one's choices for the rest of one's life. Maester Aemon remains at Castle Black during the Lord Commander's great ranging.
Maester Aemon is still at Castle Black when Samwell Tarly returns with Gilly after escaping from Craster's Keep. Aemon at first advises Sam that he swore an oath when joining the Night's Watch, believing that Sam fathered Gilly's son. Samwell corrects him that he is not the father, but that the girl is one of Craster's daughter-wives, which engenders sympathy in Aemon. Because it is too dangerous beyond the Wall, Aemon relents and offers shelter to Gilly in Castle Black for the time being. Gilly thanks him and calls him "my lord", but Aemon insists that he gave up being a lord many years ago (when he joined the Order of Maesters and left House Targaryen). With the matter settled, Aemon commands Samwell to make sure all of the messenger-ravens are well-fed, and tells him to take a quill and parchment: he dictates to Samwell a request for help which will be sent to all the major lords throughout the Seven Kingdoms, imploring them for aid against the threat of the White Walkers.
Maester Aemon is one of the five black brothers to "judge" Jon Snow for his actions during his time with the wildlings. Jon hides nothing of his actions, neither his murder of Qhorin Halfhand or his affair with Ygritte. Ser Alliser Thorne, the acting Commander of the Night's Watch, and the recently-arrived Janos Slynt believe that Jon should be executed for his actions and for the breaking of his vows, but Aemon defends him, saying that Jon was merely following Qhorin's orders and that he isn't the first to have broken his vow of chastity. Ser Alliser points out that Jon has quite literally slept with the enemy. Jon counters with all the information that he had learned while among the wildlings, such as Mance Rayder's strategy of attacking the Wall from the North and the South, and demands that they either kill him or let him go. Aemon tells him that they have no intention of executing him and Jon leaves. Alliser reprimands Aemon for having overruled him but the maester answers that he knows when a man is lying and when he is telling the truth. When asked how he acquired such insight, Aemon wryly replies that he grew up in King's Landing.
Maester Aemon is present when Olly brings news of the massacre that occurred in his village. Many black brothers present want to retaliate, but Aemon reminds everyone that there are just over 100 men at Castle Black (including himself). He insists that defending Castle Black is the priority and they cannot afford to lose a single man.
On the eve of Mance Rayder's assault on Castle Black, Maester Aemon confronts Sam in Castle Black's library. They discuss some of the propaganda surrounding wildlings before discussing Sam's feelings for Gilly. Aemon reprimands Sam for not realizing his elders were young once too. Aemon implies that he might have become King of the Andals and the First Men if he'd been willing to break his vows. He also mentions that he was sought after by a great many women in his youth – but there was only one whose advances he accepted. Although Aemon says that his memories of that woman are in some ways more real to him than Sam is, he lets the subject drop and says that he and Sam should both focus on their duties. Following the arrival of Stannis Baratheon, Maester Aemon leads the eulogy for the black brothers that gave their lives defending Castle Black. When Jon Snow speaks to Tormund, the captive wildling curiously asks why he was given medical treatment. Jon tells Tormund that it is Aemon's duty as a maester to treat anyone who is wounded, friend or foe.
Maester Aemon presides over the choosing of the 998th Lord Commander of the Night's Watch. When all of the votes are counted, it is revealed that there is a tie between Jon Snow and Ser Alliser Thorne. However, Aemon is the last to vote and breaks the tie by voting for Jon.
Jon Snow notices Maester Aemon's absence at a meeting of the Night's Watch and questions Sam. According to Sam, Aemon does not feel well and apologizes for not being able to attend the meeting. Jon tells Sam to take good care of the old maester.
Aemon is revealed to be keeping tabs on his sole living relative when Sam reads him a missive from Slaver's Bay detailing Daenerys Targaryen's recent activities. In spite of his dedication to his vows, Aemon muses with sadness at the injustice of Daenerys fighting her battles alone, with her only living relative "useless and dying" thousands of miles away. Jon Snow then arrives, asking for Aemon's advice on a plan he's been devising. Aemon bluntly tells him to "kill the boy, and let the man be born", that Snow must make the mature decisions that will save lives, rather than the immature decisions that will get people to like him.
Aemon's old age catches up to him and he takes to bed as he grows weaker, tended by Gilly and Sam. His mind starts to wander, and he imagines conversations with his late brother "Aeg" before he passes away. Sam gives a eulogy as Aemon is put to rest in the traditional way of the Night's Watch.
The longtime maester of the Night's Watch, Aemon was blind but sharp. One of the last remaining Targaryens, Aemon understood the brothers of the Watch left their family ties behind. Fond of both Sam Tarly and Jon Snow, it was Aemon who cast the deciding vote that made Jon the 998th Lord Commander of the Night's Watch.