Before Titan's Curse
Nico and his older sister Bianca were born before WWII. Hades sent them to the Lotus Casino to keep them save from Zeus(who had killed their mom Maria).
Hades had them brought out of the Lotus Casino after what they thought was a month(which was in fact 70 yrs.).
Percy, Annabeth, and Thalia meet up with Grover at Westover Hall, Maine where he has discovered 2 halfbloods together (a very rare occurrence since their scent is twice as strong together and would have attracted monsters much sooner).
Well then end up saving the 2 half bloods(named Bianca Di Angelo and Nico Di Angelo) from Dr. Thorn; a manticore, and with Artemis and her hunters, help defeat him. During the process Annabeth goes missing in action much to Percy’s devastation.
Artemis goes hunting for “the monster that will destroy Olympus” and the halfbloods and Hunters take a ride with Apollo to camp.
Nico makes Percy promise to protect his sister on her quest to which Percy promises.
When they arrive home Percy has the hard job of telling Nico of Bianca’s death…Nico then hates him and a ton of skeletons rise out of the ground. It soon becomes painfully obvious that he is a son of Hades and that the prophecy may not be about him after all. Percy then claims the prophecy for himself and has his friends vow not to tell anyone about who Nico’s Dad is.
Battle of the Labyrinth
Nico first appears in a Iris Message where he is seen talking with Minos who says that in order to resurrect his sister he needs a soul who has cheated death to trade.
Nico next appears while trying to summon his sister to which she won't come. After that he appears at the farm of Geryon. Percy makes a deal to save his life and after freeing the farm Percy asks Nico to bring Bianca who reveals that Children of Hades have the weakness of holding grudges. She tells him that the one he is truly angry at isn't Percy but rather Bianca for first deciding to become a hunter and second for dying, she then asks him not to resurrect her.
When offered to join them on their quest he refuses saying he needs time to "collect his thoughts".
Soon the group learn that Nico has gone back into the Labyrinth and has been talking to Minos again.
Nico then meets them inside Daedalus' lair. Nico then reveals that he went in to save them and that he had been tricked by Minos who starts saying his plan about trading his soul for Daedalus' and when Nico says for him to away he says he is the Ghost King. Nico then screams "I am the Ghost King" runs his blade into the ground and Minos is sent away.
When Kronos takes Luke's body he chases the halfbloods and in order to cover their escape, Nico reveals that he is a son of Hades.
After the great battle at camp, Nico eases Daedalus into the after life(without resurrecting Bianca as per Bianca's instructions) and the Labyrinth closes.
At camp Nico didn't participate in anything and stayed in the shadows...just watching. After the camp fire, Percy seeks him out and sees him say goodbye to Bianca, one last time. Nico then says he is leaving immediately and has tons of questions to ask...places elsewhere. Coaxing him didn't work cause he didn't feel welcome. Percy then gives him the Hades figure from the Mythomagic game. After that thet say goodbye to each other.
Nico then appears at the Percy's house where he comes in to talk with him about his plans on how to defeat Kronos.
The Last Olympian
He is first seen in the book in one of Percy's dreams where Nico tells him, that time is running out and that he needs to act on the plan quickly.
He is next seen with Percy and Mrs. O'Leary as they meet and talk with Mrs. Castellan(Luke's Mom).
Later, Percy is tricked and betrayed by Nico in order to get some info. Nico then rescues him and Percy is bathed in the River Styx.
Next time we see him he finds out the lawyer who got them out of the Lotus Casino was the Fury Alecto. Nico tries to summon his mom to be told by Bianca to not try. He then sees that Zeus had killed his mom and that Hades had sent them into the Lotus Casino to save their lives. He then had their memories wiped so as to remove the hurt from them.
Nico then is able to persuade Hades to come to the gods and demigods' aid telling him to overcome his weakness of holding grudges. When Hades pats Nico on the shoulder, Percy says that is the happiest he had ever seen. The Last time he is seen in the book is him using some dead people to help build the Hades cabin.
The Lost Hero
Nico is one of the people who is said to be looking for Percy.
Son of Neptune
He first appears at Camp Jupiter and sees Percy much to his surprise but he quickly gets over it. He then says to Hazel that while he does know him that Percy must regain the memories without his help.
It is heavily hinted that he has known about Camp Jupiter for a long time(even before Jason told the campers about it in Lost Hero). It is most likely his dad, Hades had told him in order for him to be Hades' ambassador to the camp(which he claims to be).
He is later seen watching the war games and then at the meeting supporting the side wanting to fight the giants and telling them how to do it.
At the end of the book it is revealed that he has gone to the Underworld to locate the Doors of Death and is kidnapped by Enemy Forces and is in Rome somewhere.
Powers and abilities
ADHD: Nico's ADHD is the product of his supernatural battle reflexes/senses. This trait he shares with all known halfbloods
Dyslexia: He has problems reading normal languages cause his mind has been hardwired for ancient greek
Fighting skills: In the beginning he only showed moderate skills and used his powers to win his fights rather than sword fighting skills. By the time of Son of Neptune his skills had improved significantly to the point that it impressed Hazel more than most of the Roman Demigods' fighting skills(something very big considering their patron god is Mars god of War and swordfighting is there thing)
Geokinesis: Like all children of Hades he has the ability to control the Earth. He has opened fissures, caused trembles so powerful that they shattered pillars of a mountain fortress, and can control the black walls of the Underworld.
Necromancy: Nico has the ability to summon the dead, make the dead sleep, has power over the furies, release those who should have died long ago. He can destroy skeleton warriors, can manipulate ghosts, and has been seen to raise a big army of skeleton warriors by himself.
Death Sense: Nico can sense when death is near or when someone has died, he can sense when a immortal(like Pan) is fading away, and can also see someone's life aura. Furthermore he can sense when someone is being judged in the Underworld.
Umbrakinesis: Nico has the ability to control shadow and darkness for various effects such as hiding and solidifying it into weapons.
Shadow Travel: He can use shadows as a means of transportation though using this for far distances tends to tire him greatly.
Induce Fear: He radiates fear and death just like Hades though to a far lesser degree.