Schyuler O'Shaughnessy, after 2 years of working under a contract in China, has come home to New York with an infant in his arms. The baby wasn't his to begin with, but he only adopted it from Apate. It's biological father was nowhere in sight, and Schyuler was the only person the goddess trusted at the time. He lied to his parents that his friend in China died shortly after giving birth, and entrusted the child to him. However, his parents, Ulysses and Josephine, did not like what he had done, and forced him to put the child, named Sealtiel, up for adoption. He has grown fond of him, yet still didn't know what to do.
He went to a nearby bar to clear his head, and that's where he met up with his ex-fiancee, Sandra Crusoe. She had broken their engagement several months back after pursuing a career in photography in Nevada. Unfortunately, she never enjoyed it, and has regretted her decision. But after talking with each other the whole night, the two finally made up and inevitably slept together a few days later.
At Schyuler's bachelor party in a hotel in Atlantic City, one of his friends hired a stripper to perform for him, who is surprisingly the goddess Nemesis. After getting drunk all night, they slept together, impregnating the goddess. Finally, after two weeks since their bachelor party and bridal shower, Schyuler and Sandra got married.
Several months of a happy married life passed, and Sandra gave birth to their firstborn son, whom they immediately named Cilan. Around that same time, Nemesis also gave birth to a demigod boy. While Cilan was in the maternity ward, the doctors then discovered that he had a severe case of respiratory distress. However, they were too late to save him, and he died a few hours after. But before the doctors could announce his passing, they were amazed to see that he was brought back to life, with no signs of disease. Truth be told, Nemesis had switched the two infants. She would bring her son to Schyuler sooner or later, and that was the right time to do it.
The O'Shaughnessy's a cut above the average American family, so one must expect behavior of the same degree, but that wasn't always the case. Sealtiel would cause mayhem anytime and anywhere, and was a pain in the neck of almost everyone. Cilan was the polar opposite, and the only one who could put a stop to his brother's trouble-making. In fact, being the more disciplined one between the two of them, he would often act as the older sibling. Cilan generally sees Sealtiel as shallow, stupid, and hard to manage; but still finds him quite fun to be with, all the while keeping a safe distance from the disasters he is causing.
Their parents, on the other hand, had started to become estranged. Schyuler and Sandra had different ways of imposing discipline upon their children; while Schyuler spoiled Sealtiel and always covered up for his nuisances, Sandra would do otherwise. The former had disliked her adopted son ever since it started going to school, and had treated it with contempt. Over time, their fights became worse, with Schyuler's work problems and Sandra's unbearable in-laws adding more rifts to their relationship. Finally, after 9 years of marriage, they decided to call it off. Schyuler kept Sealtiel with him in New York, while Sandra took Cilan to her hometown in Allentown, PA.
Cilan spent the following years of his life in apparent peace, with no fights between husband and wife to ruin his nights. He just had to attend to the needs of Sandra's aunt Lana, a sickly woman in her late 60's who has been suffering Alzheimer's and diabetes for quite some time. He also lived with Sandra's younger sister, Kathleen, who pretty much has a life of her own. His stepmother, before divorcing Schyuler, had asked the latter to provide for him while she gradually paid for Lana's hospital debts. Soon enough, the latter was sent to an elderly care home.
To be honest, Cilan missed Sealtiel a lot, and in order to mask his longing for his brother, he seemed to display a certain level of energy that is stored whenever he was at home. He channeled this energy in different ways, such as joining his school's band, going out with his friends on Friday nights, or by playing soccer and baseball. He wasn't the brightest nor politest kid in school, but he sure made up for it with his enthusiasm. He would go home late, date some girls, drink something alcoholic every now and then, but made sure he's always on check. Long story short, he was your typical teenager.
It wasn't that long before his demigod scent became strong enough for monsters to follow him. What followed after became his first monster attack, when he and Kathleen visited Lana at the elderly home. Two harpies posed as nurses began to chase after him. As he rushed outside with the old woman in her wheelchair, the next thing that happened went in a blur. The only thing that he could remember before passing out was a shawl-wearing woman bashing the two harpies to dust with a bronze-like walking stick. Whether it was Lana or somebody else, he didn't bother to know.
News came of Sealtiel's sudden disappearance after setting fire to his school, and Cilan felt the need to go to New York and find him. Sandra was absolutely against him, accidentally spilling out the truth that his brother was adopted. While it angered him even more, he decided to play along by acting that he now hated Sealtiel in order to calm Sandra down. But later that night, he packed his stuff and left.
But as he went his way from the backyard, a cyclops attacked him. This woke up Sandra and Kathleen, and found Cilan being knocked out by the monster. When it turned its eyes on them, it was knocked out as well by none other than Lana. She explained that she was the one who rescued Cilan and Kathleen during the harpy attack, revealing her demigod status as a child of Apollo. The shelter was in fact a home to some veteran demigods, with disguised nymphs attending to their needs.
Sandra, open-minded yet still a tad bit bewildered, had permitted Cilan to train at Camp Half-Blood during the summer. Upon arriving, he was claimed by his mother, Nemesis, and was surprised to know that Sealtiel was also there. While a very happy reunion followed soon after, their meetings were very sporadic. Sealtiel had quests every now and then, and Cilan had his own work to do. Once in a blue moon it was, but they were able to relive the times they had together.
It was fun while it lasted, until Sealtiel went home from his quest in France with a severe case of leukemia, the second time he had the disease. Cilan was back in Allentown when he caught wind of the news, and promptly went to the hospital his brother was admitted. When he got there, he heard shouts and cries coming from his room. Eavesdropping, he heard Sealtiel saying that he has been disowned by Schyuler after disappearing for so long, and doesn't even care if he died. Another voice was heard, which came from Koreen Trent, the one who brought Sealtiel to the hospital.
Sacrificing the time he gave to visit his brother, he went straight to the O'Shaughnessy's home to give his father a good beating. If it weren't for some of the butlers and maids holding him off, he would've killed him. He can kiss his son's ass goodbye, because Cilan didn't care about him anymore. He would never forgive him for what he had done.
On his way back to the hospital, he had spotted a figure soaring across the sky with another one clinging helplessly onto it. As soon as he knew it was Sealtiel being taken away, he immediately went after his abductor, Icarus Willow, a member of the Broken Covenant. A vicious fight soon followed. However, Cilan was overwhelmed by the power of the child of Kratos. In the end, he was badly defeated, and Icarus escaped, taking away Sealtiel with him to whereabouts unknown.
Cilan woke up a few days later, and he quickly visited the Hades Cabin to see if his brother was already dead. He found out that he was still alive, but was definitely missing. After his fight with Icarus, he decided to stay at camp to train more; when he's strong enough, he would go look for his brother. Ever since then, he went to Hades' Cabin everyday, with high hopes that he wasn't too late.
What he didn't know was that Sealtiel returned to camp and was "cured" of his sickness and wounds by Apate , in exchange for his memories with Koreen. But he quickly left and has once again gone missing, probably thirsty for revenge.