Ren Allsbrook is a Terrasenite and a member of the former court of Orynth. He is the heir of Allsbrook, but had to flee with his grandfather when Adarlan invaded. His parents were executed during the invasion. He and his grandfather are part of a rebel movement in Rifthold.
Ren is depicted as being about eighteen to twenty years old. He is tall and broad-shouldered with shoulder-length black hair, a scarred face and brown eyes. His scar slashes though his eyebrow and down his cheek.
Ren can be arrogant and often lets his head rule his heart, but he is grateful and also kind to those who he befriends and trusts. Ren is also brash and will run headfirst into something before thinking it through properly, as we see when we charges at (the formerly know) Celaena. Ren is caring and protective of those he trusts and loves.
Ren makes his first appearance in Crown of Midnight, although his identity is not revealed until Heir of Fire. Ren and his grandfather are part of Terrasen's rebel group in Rifthold, and they worked with Nehemia before she was assassinated by Grave.
Ren and his rebels were the group that captured Chaol and held him hostage in a warehouse in order to get Celaena's attention. Celaena and Ren even fight during her rescue and Murtaugh himself begs her to stop the carnage. However, at this point, neither of them realizes who the other is and Ren doesn't realize she is actually his queen until he meets her in Empire of Storms.
During Heir of Fire, Aedion reunites with Ren and Murtaugh when he arrives in Rifthold. They are members of the rebel movement in the capital and they are present when Chaol reveals to Aedion that Aelin is still alive.
Ren and Murtaugh continue to work with Aedion against the King of Adarlan and after Ren is injured by the king's guard, they even stay in Aelin's warehouse apartment.
After Aelin kills General Narrok in Wendlyn, Murtaugh sets off to alert allies and Terrasen that their queen is returning. Ren stays in Rifthold until Chaol tells him to go to Terrasen, deeming it unsafe for him to remain in the city while the King is targeting those with magic in their blood.
He does not meet Aelin at all throughout Heir of Fire, and Aedion even refuses to tell him of her secret identity (Celaena Sardothien).
Ren meets Aelin in person for the first time in ten years in Empire of Storms. Ren has come from Orynth with Lord Darrow and his grandfather to meet with Aelin and discuss plans for Terrasen's future. When Darrow tells Aelin that she will not be recognized as the rightful Queen of Terrasen, Murtaugh is shocked, so it can be assumed Ren was also unaware that this would be happening.
As Aelin and Aedion are planning to leave for Ilium, Lysandra talks to Ren and Murtaugh and asks them to take Evangeline to Orynth with them, so she will be safe. They agree and when Ren notices the scars on Evangeline's face - so similar to those on his own - he softens a bit.
He makes it clear however, that he's not completely on Aelin's side and he does not yet forgive her for abandoning Terrasen during the last ten years. Aelin understands his aversion and distrust and does not hold it against him.
Aedion tells Ren and Murtaugh to get the Bane ready to push from the North - against Darrow's orders - but after Aelin, Aedion and Lysandra leave, they are not seen again.