The recurring idea of being the seventh wheel associated with Leo may have something to do with his eventual sacrifice, as seven means 'ghost' in Chinese as stated by Frank's grandmother.

When Leo accidentally polishes Buford with Windex instead of Lemon Pledge with extra moisturizing formula, Buford runs off with two syncopations essential to building the engine for the Argo II. Leo, in a panic, asks for Jason and Piper's help to retrieve them. He explains the situation of how in about an hour, dangerous chemicals will mix in the engine blowing up the forest portion of Camp Half-Blood without the syncopations. With everyone caught up, the three friends journey into the forest in search of Buford. They then encounter Maenads on their journey and Leo feigns to be the Greek god, Dionysus, to please the Maenads. Suspicious, the Maenads figure out the truth and chase Leo and his friends until Leo comes up with an idea. Leo sends Jason to go find Buford, while he and Piper run towards Bunker 9. Once they reach Bunker 9, Leo distracts the Maenads with a party while he sets up a trap to ensnare them. Leo finally sets off the trap which drops a net (based on the original net which Hephaestus used to catch Ares and Aphrodite together) on the Maenads. Jason just in time arrives with Buford and Leo fixes the syncopator in the engine of the Argo II, thus narrowly preventing a massive explosion at Camp Half-Blood. Later Chiron and Argus arrive and decide to deport the Maenads away. At the end, when the gods are called upon to fight the giants, Jason and Zeus fight Porphryion, King of the Giants, with Zeus shooting his Master Bolt to kill him. Jason manages to make a good impression on his father. Later, when Zeus says that Apollo must take the blame for what has happened, Jason risks his wrath by saying that appointing blame doesn't solve anything and calls Zeus's decision to punish Apollo for Gaea's waking unwise, but Artemis placates Zeus's anger and Jason notes that he might have pushed his father too far. Leo is the only person to physically appear in all 5 books of The Heroes of Olympus as he was in a video in The Son of Neptune. After Leo ran away for the sixth time, when he was fifteen, he was caught by a truancy officer in New Mexico. He was taken to court and sentenced to go to the Wilderness School in Nevada, a correctional facility for delinquents, where he would stay for almost half a year. While Leo was there, he was protected by Coach Hedge and befriended Piper McLean, with whom he shared secrets with: Leo learned how Piper's dad was a celebrity and how she stole a BMW, and Piper learned how Leo ran away six times. Leo hated being at the Wilderness school, likening it to a prison, and would have to run ten miles a day and be pushed to his limits. It also forced Leo to go on several forced marches. However, he liked to mess with Coach Hedge. Since the House of Hades, Jason had nightmares about Gaea. Sometimes in Epirius with Clytius, some with Gaea destroying Camp Half Blood, and one with Lupa killing him after revealing her true form and his mom leaving him. In the last nightmare, he would be scared and wake up with tears. He was also very worried about Reyna, Nico, and Hedge. Especially Nico, and he would pray to his father to help him after everything he'd been through.Once the Argo II made it to Delos, Leo wanted Hazel and Frank to come with him because he trusted the two more than the others to know about his suicidal plan to sacrifice himself. Hazel was very upset when she heard the news, and a tear fell down her cheek. Leo told Hazel that she was the only one who could cover for him, since Sammy knew how special she was. He said that he blessed him when he was a baby because he knew that Hazel would help him, and that everything was leading up to it. Hazel burst into tears and hugged Leo. Hazel said that she hated the plan, but she would do it for him. Hazel ended up making. Fake mist version of the Physician's Cure and gave it to Piper. Jason has had daily audiences with Auster, the Roman god of the south wind, in connection with the Seven being able to proceed on their way to Epirus. Jason got very tan during his stint in Africa but very annoyed at Auster. After five days, Nico spoke to him and asked if there was any word from the king. Nico told Jason that they needed to leave soon and he sensed a bad feeling about the doors. He then said he had to convince Auster to help them, since he suffered too many humiliations. Jason then tried to discuss what happened in Croatia, saying he understood him and no one would judge him, but Nico grew angry, saying he doesn't belong anywhere and he would never understand since he was everyones golden boy, but no one ever accepted him except for Bianca, with no say in how he felt. Finally, Jason told Nico to take a chance and trust him, and that its better than hiding, and even called him out for always running away and hiding, but Nico snapped and said he would help him then leave forever. Jason began to think being friends with Nico was more trouble than its worth.

Zeus | Hera | Poseidon | Hades | Ares | Demeter | Athena | Apollo | Artemis | Hephaestus | Aphrodite | Hermes | Dionysus In September, after Jason and Piper received a parchment scroll from Leo, they spent the entire month looking for him, until Chiron urged them to get on with their education. Jason still hoped during those months that Leo was still alive. As for Leo, in both The Hidden Oracle and The Dark Prophecy, he expressed interest in seeing Jason again. Cupid began to taunt Jason, but Nico told him to leave him alone and that he knows its really him he wants. Cupid extorts a confession from Nico as the price of giving him and Jason the scepter: Nico had fled Camp Half-Blood after Bianca's death not just because he felt rejected by the other campers as the son of Hades, and not just because Percy had let him down by failing to keep Bianca safe—he had romantic feelings for Percy, which he could not deal with or let others know. Jason began to understand Nico and realize he was wrong about him, and wanted to be his friend. Seeing this, Jason tries to tell Nico that he would keep it a secret but comments that everyone would still stand by Nico if he told them. He also said out of everything he had seen, what Nico had done was the bravest. However, Nico tells him that he's fine and no longer has feelings for Percy, though this is questioned later on in the book. They then returned to the ship. The Lost Hero | The Son of Neptune | The Mark of Athena | The House of Hades | The Blood of Olympus After that, Percy and Leo were friends with no problems and they defeated Nike with the help of Frank and Hazel. Leo became sarcastic and joking with Percy, calling him "Water Boy", which he told him not to call him, then calling him "Aquaman", which he hated more. Leo was also miffed and asked him why he had to destroy the plumbing when he did. Percy also joked that if a statue engulfed people in fire, he should send Leo, and Leo jokingly said he loved him too. Leo was also grateful when Percy got everyone in the crew, including him, gelato, and said that it made his whole day better. Leo also joked about how Percy's zodiac sign was Leo's name. However, Leo kept his suicidal plan to sacrifice himself from Percy. Percy was devastated when Leo died, and was initially angry at Hazel and Frank, but his anger subsided when they cried and agreed that it was a plan Leo would've done. Percy was hopeful that Leo was alive and said that if he lives, they can take turns strangling him.Eventually, Lupa took him on a trip south to Camp Jupiter, where she showed him around Camp. Jason was offered a place in the First Cohort, but he denied it, and decided to join the Fifth Cohort to restore honor to it after what happened to Michael Varus. But he also joined it to try to alter his destiny as the son of Jupiter. In The Mark of Athena, Piper wasn't angry at Leo when he fired on Camp Jupiter, and agreed that it couldn’t have been him, and could recognize the cold feeling he felt. Piper also figured out that Leo was possessed by Eidolons. Leo thanked Piper for not hating him after firing on the Romans, and Piper said it was okay, and he wasn't in control of himself. Leo said she didn't have to stand up for him, and Piper said of course she would, he was the annoying little brother she never had, showing how much she cared for him. Leo also teased Piper and said she had power ranger pajamas, and Piper said they were Cherokee Eagles. When Leo went missing in the Atlantic Ocean, Piper didn't sleep that night and looked in visions in Katoptris to find him. When Leo returned, Piper cried and freaked out, jumping in the ocean to greet him. She gave him a sisterly kiss on the cheek, with surprised him, and asked where he was. Piper wasn't afraid to lecture Leo, lecturing him when he didn't want to save Nico di Angelo and made Hazel upset. Leo saved Piper in Rome later on the quest, along with Percy, Jason, and Nico. The four arrived at Olympia and he searched the museum with Percy. They sat on a bridge overlooking the Kledos river, and it was very awkward for Leo to hang out with Percy due to their history with Calypso, and him being a survivor of Tartarus. Percy noticed that Leo was looking at him angrily and asked him why, and said that they should talk. They were interrupted by Hazel and Frank, who could not find Nike. The four looked around the ruins, which Leo was not impressed by, annoying Hazel. The more into the ruins Leo went, he became more uneasy, thinking about his past. They stopped at the Temple of Zeus, and Leo realized that was where Nike most likely was. Leo and Percy managed to summon Nike by saying that Adidas was better, and Nike and appeared. Leo asked Nike what was going on, and she split into her Roman form, Victoria. Both goddesses said that they had to fight Greeks versus Romans: Leo and Percy versus Frank and Hazel, and has to fight the Nikai. Leo tried to talk her out of it, but it was no use. The Nikai chased them and they had tone hide. Percy, Frank, and Coach Hedge embark into Atlanta while Annabeth attempts to cheer up Hazel. Jason, Piper, and Leo are interrupted from their lunch to disembark, causing Leo to cry out if he can ever just eat a normal lunch. Percy tells him to head to Charleston where they can find a map to the Mark of Athena. Percy and Frank recount how they had met Phorcys, Keto, and the imprisonment of wild animals. They had also been imprisoned but escaped causing Leo to wonder if he's worth as much as two or three Franks. Frank, meanwhile gets himself trapped in Chinese Handcuffs, much to Leo's amusement. He changes into a lizard to escape which Leo jokes about. Percy then states that the map leads to the Mark of Athena causing them to wonder what it is. He would be at five more foster homes after that. Some foster homes would last longer than others and he would make friends, joke around, and pretend that nothing bothered him, but he always ran away sooner or later. At the foster homes he was at, he was frozen out and ignored, and they often treated him like they didn't need him. He even had an abusive foster mom, Teresa. From being in foster care, and in rough schools, he learned to tell when fights were going to get ugly. He would have nightmares and hug his pillow, which he thought was embarrassing in the foster homes.