In "The Titan's Curse", Nico is a composed kid who loves a trading card game called Mythomagic. However, since his sister's death in "The Titan's Curse" at the hands of a defective Talos, he has a hateful resentment toward Percy. He later thinks Mythomagic is a stupid game, and burns all his cards in a fire. He becomes more sullen and solitary, saying that "only the dead respect him... and only out of fear". He believes that he will never be accepted with the living, but forgives Percy after Bianca tells him that it wasn't Percy's fault that she died. He also accepts the small figurine of Hades that Bianca wanted him to have before she died in "The Titan's Curse". In the fifth book, he brings Percy to his father to get information that proves to be the least his father can give, and his father ends up putting Percy in a dungeon. However, Nico rescues Percy from the dungeon and takes him to the River Styx where Percy becomes almost invincible. In the end of "The Last Olympian", Nico convinces his father, Hades, to help the other gods fight in the war against the Lord of Time, Kronos, and his army, and plays a major role in the victory. The relationship between Percy and Nico becomes close for they both might have been the child of the Great Prophecy.
Nico Di Angelo's fatal flaw is holding grudges, as Bianca tells him. In his case, it can lead to not wanting help or not asking to be saved in deadly situations because he does not like someone. Examples are his not wanting to be helped by Percy, even when Geryon was going to sell him to the Titans and not wanting to go back in the Labyrinth with them. Bianca shares this with him, as she also had the same fatal flaw in life, although this did not lead to her death. Bianca also says that holding grudges is the demigods of Hades' fatal flaw. In the fifth book, Nico tells his father, Hades, that he is holding a grudge that could end up killing the Olympians and the campers.
In "The House of Hades", Nico is forced by Cupid to reveal that he had had a crush on Percy Jackson. This reveals that he is homosexual. In The Blood of Olympus, he later says that he does not feel that way anymore about Percy and that he had been young and impressionable at the time, (by saying Percy is: "not his type) and is seen to be potentially crushing on Will Solace (head counselor of the Apollo cabin).
In The Trials of Apollo: The Hidden Oracle, Nico is in a romantic relationship with Will and receives the enthusiastic support of Apollo (Will's father) for their relationship.
Nico is ten years old and first appears with his sister, Bianca, in a military school called Westover Hall in Bar Harbor, Maine. They are sought out by Grover Underwood, the satyr, who believes them to be two very powerful unknown half-bloods. Nico seems to be very interested in Mythomagic, a game about Greek mythology. When his sister Bianca accepts Artemis's proposal to become a Hunter, Nico gets angry at her for "abandoning" him. Percy finds Nico eavesdropping on Bianca and Zöe Nightshade (one of the Hunters of Artemis, a daughter of Atlas), and Nico makes Percy to promise to keep his sister safe. He does not appear again until the end of the book, where Percy explains to him that Bianca died to save the others on the quest. Nico creates a large crack in the ground to swallow up the skeletons that were chasing Percy. The he immediately turns on Percy, telling him he hates him, and reminds him of the oath he took to keep Bianca safe. Nico is last seen running into the woods at night, after screaming at Percy.
Nico is supposedly eleven years old in this book. He is first seen talking with an unknown ghost who is trying to tell Nico how to trade his sister's soul for another soul, when Percy is sent a collect Iris Message sent by Bianca Di Angelo, Nico's sister. It is revealed in this book that Nico can summon the dead. Nico is then seen when Percy, Annabeth, Grover, and Tyson visit Triple G Ranch, where Percy then saves Nico from being sent to the Titans. Nico, still angry at Percy, finally understands that it was Bianca's choice to go into the Talos robot, and that he shouldn't be mad at Percy. The others leave Nico at the Ranch, where he supposedly meets King Minos (in ghost form) again, who convinces him to go back into the Labyrinth to supposedly save Percy and the others.
He is next seen being dragged into Daedalus' workshop by one empousa and two Laistrygonians, in chains, after being tricked by King Minos. He escapes with Percy, Annabeth, and Rachel Elizabeth Dare, who have to go back into the Labyrinth to find Grover and Tyson, who had stayed behind to look for Pan, god of the Wild. Along the way, they see Kronos in the form of Luke, and Nico, in a desperate attempt to save everyone, shows his true powers, and makes it known to Kronos that he is a son of Hades. They meet the wild god Pan, who gives everyone except for Nico a cryptic future reading and then vanishes. After they get out of the Labyrinth, a full-fledged battle erupts between the Titans and Camp Half-Blood, in which Nico summons a dozen undead soldiers to fight the dracanae, passing out in the process. He is revived with nectar and ambrosia, and he states that summoning that many soldiers required a lot of energy. After the campfire, he is seen talking with the ghost of his sister Bianca. He then tells Percy, who came out to see what was going on, that he did not belong at camp, but instead in the Underworld; he would train there instead. He also goes to search for his mother. He is last seen on Percy's apartment fire escape giving Percy news on Daedalus and a way to beat Luke for good. Before mentioning it, Nico is distracted by Percy's blue birthday cake, and Percy invites him in to talk.
Nico is twelve years old in this book. His theory to gain an edge over the Titans is to make Percy invincible by having him bathe in the River Styx, similar to the myth of Achilles. It is shown that after Bianca died, Hades feels he has no competent child, repeatedly mentioning that Bianca could do a better job at fulfilling their father's demands than Nico. It is also revealed that the lawyer who removed the children from the Lotus Casino was Hades' lead Fury, Alecto, in disguise.
Nico is in about the quarter of the way in the book, he brings Percy Jackson to May Castellan's house, and Hade's palace to talk with Hades but instead to Percy Jackson gets put in the dungeon and rescued by Nico. He fights but comes a little late to help with the Kronos war, at the end of the book he talks about Rachel Elizabeth Dare being missing.
Nico appears in the beginning of the book, He is visiting his half-sister Hazel, who was showing Percy (after losing his memory) Camp Jupiter at the time. Nico lies to Percy about knowing him after he finds out that Percy's memories have been stolen and simply says that he's an ambassador for Pluto at the Roman camp. It is later made clear that this was to preserve diplomacy (since Greeks and Romans didn't get along) and not interfere with Hera's plans - rather than a malicious urge.
Though Nico was inside of a bronze jar (after getting captured) during most of The Mark of Athena, he does get rescued by Piper, Jason and Percy. Nico also explains to the others where and the doors of death are and how to close them. When Percy and Annabeth are holding on for dear life at the edge of a chasm that lead to Tartarus, Nico desperately tries and fails to save them before they fall into Tartarus, promising he would lead the others to the other side of the doors of death.
The Hidden Oracle
He dosn't do much just talking, not much action with him in this book.