Grenn is a ranger in the Night's Watch, he joined and trained at the same time as Jon Snow.
Grenn is a new recruit into the Night's Watch. He is strong but slow-witted. Grenn's father abandoned him, leaving him outside a farmhouse when he was only three years old. He grew up on the farm, and had a relationship with a neighboring farmer's daughter in his youth
Grenn is a recruit in the Night's Watch training at Castle Black. Grenn, Rast and Pypar are directed by the Master-at-Arms Ser Alliser Thorne to test Jon Snow's skills in combat. Jon breaks Grenn's nose in the fight. Annoyed by Jon's superior attitude, the three of them threaten Jon in the armory, but are encouraged to leave him alone by Tyrion Lannister. Tyrion suggests to Jon that he remember their deficiencies in training compared to his own privileged upbringing. Jon later starts training Grenn and the others in the art of combat, earning their trust
Grenn and Pypar have become friends, with the latter teasing Grenn on his slow wit. Grenn agrees to Jon's plan to go easy on the new recruit Samwell Tarly. During practice, he urges Sam to hit him, and promptly falls over screaming from a small tap to explain why he can't continue the fight. Ser Alliser is unconvinced.
Upon completion of his training, Grenn is assigned to serve amongst the rangers. Pypar and Grenn are among the men who witness the arrival of the bodies of Othor and Jafer Flowers to Castle Black. The dead men return as wights and Jon saves the Lord Commander from one of them. Grenn, Pypar and the others inspect Jon’s new sword, Longclaw, a reward from the Lord Commander. Grenn, Pypar and Sam ride after Jon when he chooses to abandon his post at Castle Black after the death of Eddard Stark. They talk to him, reciting their oaths, and convince him to return.
In the far North the great ranging of the Night's Watch under Lord Commander Jeor Mormont traverses the Haunted Forest. They have passed six abandoned wildling villages and have been travelling for months. Jon Snow passes the sledge housing their ravens. He asks Samwell Tarly if he is having a hard time and Sam responds that nothing has killed him. Grenn is trying to push the sledge through the mud and complains that Sam’s weight damaged it. Sam counters that Grenn offered him a ride and Grenn says that he only did so because he was fed up of Sam's complaints about his blisters. Eddison Tollett watches the bickering pair. Jon dismounts and leads his horse towards the wooden hall known as Craster's Keep. Edd stands beside him and deadpans that he was born in a similar place but that he has now fallen on hard times. Sam joins them and notes the women working around the hall, asking if they are girls. Sam says that he has not seen a girl for six months and Edd warns the younger men not to speak to them. Grenn wonders if Craster does not like people speaking to his daughters and Edd says that they are also his wives, explaining that Craster marries his daughters and breeds more daughters with them. Sam and Grenn state their disgust and Edd counters that while all the other wildlings in the area have disappeared Craster is still present so he must be doing something right. Jon wonders what happens to Craster's sons. Lord Commander Jeor Mormont meets with Craster and learns that the wildlings have gone further north on the orders of King-Beyond-the-Wall Mance Rayder. The Lord Commander accepts Craster's offer of shelter and his condition that the men do not touch any of his wives
Sam, Edd, and Grenn are peeling potatoes outside Craster's Keep. Edd says that if the gods wanted them to have dignity they would not have made them fart when they died. Grenn is incredulous and Edd tells him the story of his mother's death. He claims that he held her hand as she passed and that she farted long and hard, blowing a raspberry to underscore his assertion. Sam notices Gilly passing with two rabbits and observes that it is greedy for Craster to have so many wives. Edd deadpans that they were having a serious discussion. Grenn points out two other women walking past. Sam says there is nothing like the sight of a woman walking away and Grenn replies that he prefers watching them come towards him. Grenn tells the others that he grew up with, and later slept with, a milkmaid named Violet. Sam is fascinated and says that he wishes he grew up on a farm. Edd sends Sam for more vegetables. Sam crosses paths with Craster's wife Gilly and becomes infatuated with her.
Craster catches Jon following him into the woods with a newborn boy. Craster incapacitates him and demands that the Watch leave. The ranging marches to the Fist of the First Men. Jeor tells Jon that Qhorin Halfhand cannot have arrived to meet them or he would have sounded his horn. Grenn, Edd and Sam trudge behind them. Jon and Jeor discuss the Halfhand's reputation as a skilled ranger. Sam says that the scenery is beautiful and that Gilly would love it there. Edd tells Grenn that there is nothing more sickening than a man in love.
Having reached the fortified summit of the Fist the rangers prepare their camp. They are digging through the snow and unloading their gear. Ghost has rejoined the party and watches the men work from atop a mound of snow. Sentries dot the edges of the summit. Sam is amazed at being in such an ancient place, telling his companions that it was fortified by the First Men in the Long Winter thousands and thousands of years earlier. He considers all of the major historical events that have happened since in wonder including the invasion of the Andals and the conquering of Westeros by House Targaryen. Edd implores Sam to stop talking. Sam asks what his companions think the first men were like. Edd suggests that they were stupid, saying that no clever man finds himself in a place like the Fist. Jon suggests that their distant ancestors were afraid and came to the First to escape something. He says that he does not believe that it worked
A horn sounds, halting the rangers. Grenn listens intently and wonders if it is a warning about the Wildlings. Jon pauses before saying that a single blast is for rangers returning while wildlings would have triggered two blasts. Edd notes the horrible moment after one blast on a horn when you must stand and wait, wondering if a second blast will announce foes. Sam adds that three blasts are used to signal White Walkers. Jon turns to look at him and he explains that it has been a thousand years but the horn is only blown three times for White Walkers. Grenn asks how Sam knows if so much time has passed. Sam begins to say that he read it in a book but Edd pre-empts and mimics his answer. Jon says that he can see Qhorin approaching. Edd says that they will live another day, punctuating his monotone with a sarcastic cheer.
Qhorin arrives and reports sighting lookouts in the Skirling Pass. He takes a small party of scouts ahead to surprise the lookouts, Jon is among them. They do not return and are feared dead. Grenn discovers a stone tablet while digging latrines. Beneath it is a cache of weapons wrapped in a Night's Watch cloak. Sam identifies them as being made of Dragonglass.
Grenn, Edd, and Sam are gathering animal droppings that will be used to keep fires burning. Grenn complains that he be believed they would be fighting Wildlings and people shouldn't live in a place where they have to burn animal droppings to keep warm. Edd sarcastically states to tell him if they spot a tree anywhere in the vast amounts of snow. While listening to Sam talk about Gilly, the group hears three blasts; the signal for White Walkers. Grenn and Edd run back to the Fist, but accidentally leave Sam behind.
Grenn survives the White Walkers attack on the Fist of the First Men, and regroups with the remaining survivors as the ranging attempts to pull back to the Wall. While marching back to the wall with the others, he notices that Samwell collapses from fatigue. Samwell is upset that Grenn and Edd left him when the White Walkers and the wights came, but they both help him back to his feet.
Grenn arrives with the rest of the survivors at Craster's Keep, where Craster reluctantly allows them to stay. He eats inside with the rest of the survivors and notices Gilly's cries of pain while she is giving birth. Later, Grenn and Edd are shoveling pig-shit while listening to Rast voice his distrust of Craster. Rast firmly states that Craster will hand them all over to the White Walkers, but Grenn and Edd refuse to listen.
Grenn, Edd, Samwell, Rast, and the rest of the survivors stand vigil lead by Lord Commander Mormont over a deceased ranger named Bannen. Grenn states that he did not know a broken foot could kill a man. Later, Grenn is present when the Mutiny at Craster's Keep occurs. He attacks Karl after Rast stabs Lord Commander Mormont in the back.
While the brothers of the Night's Watch at Castle Black discuss the wildling threat, a single horn blast is heard. It is revealed to be Grenn and Edd, who have survived the Mutiny at Craster's Keep. Alliser Thorne questions why it took them so long to return. Grenn reveals they were held captive by the black brothers who betrayed Commander Mormont. Grenn also states that Karl is the one who is leading them and they are content with staying at the keep, eating Craster's food, and raping his daughter-wives.
Jon Snow spars with Grenn in front of new recruits at Castle Black. Grenn helps in demonstrating the wildling strategy of wielding a weapon in each hand, which Jon learned while he was with the wildlings. Shortly after, Jon is dismissed by Alliser Thorne because only Rangers like Grenn should be training new recruits, not stewards. When Jon Snow's expedition to capture or kill all the Mutineers at Craster's Keep is sanctioned by Alliser Thorne, Grenn calls the brothers in the hall to attention and is the first to volunteer to go with Jon.
Before the raid begins, Locke scouts the keep and believes that taking down the mutineers should be easy. However, Grenn warns them that their leader, Karl, is probably the most dangerous. As the former top-paid cutthroat in Flea Bottom, Grenn has seen what Karl can do with a knife. Grenn fights alongside his fellow black brothers during the raid on Craster's Keep. After all the mutineers are slain, the casualties are taken into account. They initially find that four loyal brothers died fighting the mutineers, but it becomes five when Locke's dead body is found. Grenn wonders what could have killed Locke in such a gruesome manner.
Along with Jon and Edd, Grenn returns to Castle Black and is greeted by the other brothers of the Night's Watch. He sits in as Jon argues that they should close up the tunnel.
Grenn is furious when he learns that Mole's Town has been attacked and members of the Night's Watch were butchered. Edd argues that all black brothers were under orders to remain at Castle Black. Grenn wants to retaliate, but Jon Snow reminds him that is exactly what the wildlings want. Grenn wonders how it will be possible for Night's Watch to defend the realm against a foe that outnumbers them 1,000 to 1 if they can't even defend Mole's Town.
Grenn is on top of the Wall when the wildling attack begins. While loading the ramps with barrels filled with oil, Grenn accidentally drops one off the Wall. Alliser Thorne angrily drills his men on properly following orders when another horn blows from down below at Castle Black, signaling the unanticipated appearance of Tormund and Styr's band of wildlings at the castle's southern entrance. Alliser has to go below as his leadership is needed there, entrusting command of the men guarding the Wall to Janos Slynt. When Grenn notices Slynt's incompetence, he pretends that Alliser orders Janos to go down to Castle Black. Later, two giants and a mammoth attempt to break through the gate. After the mammoth is driven off by the fire from barrels with burning oil and one giant is killed by a ballista, the remaining giant, Mag Mar Tun Doh Weg, manages to push the outer gate up to pass. Jon Snow orders Grenn to defend the inner gate with five other brothers of the Night's Watch. Arriving at the inner gate, one man cowers when faced by the giant charging at them, but Grenn manages to convince him to stay by reciting the Night's Watch oath. The black brothers ready their weapons just as the giant rams the inner gate. They successfully kill the giant and hold the gate, but all of the black brothers are slain in the process. Their bodies are discovered by Jon and Sam after the battle and Jon kneels down and closes his eyes.
When Jon speaks with Mance Rayder after the battle, Mance mentions the giant that was slain by Grenn and his companions. He tells Jon that the giant was known as "Mag the Mighty", a king among the giants from a bloodline before the time of the First Men. Jon tells Mance that Grenn came from a farm. Jon and Mance drink to their memories. After the arrival of Stannis Baratheon and his forces, Grenn is among the numerous bodies of fallen black brothers burned in Castle Black's courtyard
A strong but slow-witted recruit who joined the Watch at the same time as Jon Snow, Grenn initially resented Jon's arrogance. The two became friends when Jon began teaching swordsmanship to him and Pyp. Grenn died during the battle of Castle Black, but not before holding the tunnel gate against the wildlings.