|“||So? What are you going to do about it? Throw your mashed potatoes at me? I'm consumed by terror.||”|
– Seth Diodoros, Half-Blood
Not much is revealed about Seth's life prior to the book. Like Alex, he had his life monitored by Apollo until a certain point. But in "The Return", a book in the spin-off Titan series, he disclosed to Josie that his last name is Diodoros.
In Apollyon, while connected with Seth, Alex learns that his mother name was Callista. He says that she was very beautiful, "tall and blond, regal like a goddess," but also "cold and unapproachable". He said that she would always give Seth hateful stares. He felt like he was just a trophy being showed off due to the fact that he is the prized Apollyon. In the same book, he also states that he was born on the island of Andros, which is also Alex's last name.
He has stated that his lineage is from Ares, although all the Apollyons before him (save Solaris) were from Apollo.
We also know that before coming to protect Lucian in the first book, Seth had an account of twenty-five daimon kills.
Seth is cocky, funny, and very entertaining, but he's also annoying and big-mouthed. It's also noted that he's a player and had never been in a serious relationship before. This might indicate a problem with letting people in due to his upbringing with an unloving mother.
Seth is described as having blonde hair that is usually pulled back in a ponytail, amber/gold eyes, has a gold complexion, and perfect facial features. He sometimes puts his hair down, though it is not frequent. He is also described as tall, being around Aiden St. Delphi's height of 6'5'. Seth is the first Apollyon in his generation and is nineteen years old (said in Pure).
Alexandria Andros: Seth and Alex have a complicated relationship through out the entire Covenant series. At first, Seth was bitter towards Alex, seeing as she was the reason he was pulled out of the field. They soon work it out and get along. When Alex is discovered to be the second Apollyon, Seth starts to become more possessive of her because everyone believes that she is fated to be with him. What Alex didn't know was that for Seth to become a god killer, he would have to drain her of her powers and energy, most likely killing her.
In Pure, Alex has a lot of nightmares and doesn't sleep well, which also affects Seth because of their connection. Seth starts coming to her room, entertaining her the whole night and sleeping along side with her, as the connection between them soothed the nightmares away. After Caleb's death, Seth became protective and caring of her, stopping Marcus and Lucian from screaming at her. He states that it would only make her feel worse and acts as a guard for her. When she was locked up in her room grieving, he didn't allow anyone to come close to her, whether they were Guards, Lucian, or even Marcus and Aiden.
When they travelled to the New York Covenant for Alex's testimony, they spent hours together training. After the annual Fall Ball, Seth and Alex go for a midnight swim. They both almost kiss, after which Seth realizes Alex was thinking of Aiden and gives her a choice; to try something that will never work out, referring to Aiden, or try and see what might happen between them; no labels, no promises and no titles. They both kiss three times in this book and have a hot make out session while Alex was on the Brew. In the end, she chooses to be with Seth.
Through out Deity, Aiden and Alex try to figure out a way for her to keep her powers, but it is almost impossible because the power transfer takes place when ever they touch. Aiden realizes that the only way to save Alex is to keep her away from Seth, so he gathers together a small group of trusted people and leaves. At the end of Deity--also on Alex's 18th birthday--she becomes a full-fledged Apollyon and has a strong connection with him, causing her to believe that his beliefs are hers; that is, even when it is the opposite of what she actually believes in. In Apollyon and Elixir, Aiden tries to find a way to break the connection between the two Apollyons, but Apollo tells him that the only way for that to happen is for Alex to do it herself. In the middle of Apollyon and end of Elixir, Alex breaks the connection and goes back to being herself.
In Sentinel, it is quite obvious that Seth still has feelings for Alex. However, he feels more sorry about the fact that he put her and the others in harm after he returns back to normal and the connection is broken. He decides to make a deal with the gods after Alex dies for him, which allows Aiden and Alex to both become immortal demigods that will live together forever. He feels upset about the fact that he wasn't supposed to be the original Apollyon, but that Alex was supposed to be. This is because he was formed due to Ares, while the other Apollyons from the past generations--like Alex--have been descended from Apollo.
In the Titan series, Seth appears to have become good friends with Alex again after he heals from his past heartbreak. He even introduces Alex and Aiden to his and Josie's son, Leon.
Aiden St. Delphi: Seth and Aiden have a rivalry through out the Covenant series for Alex's affection, but Seth has always had the upper hand. This is because it is believed that Alex and he were destined to be together due to Alex becoming an Apollyon on her 18th birthday. This is also because Aiden was a pure blood, making a relationship between Alex and Aiden forbidden. Seth and Aiden later become friends after a mutual understanding. Aiden understands Seth's struggles and is thankful that he made a deal to devote his life to the gods, all so that Aiden and Alex can be together forever as immortal demigods. In the spin-off Titan series, Aiden even meets Seth and Josie's son, Leon. They appear to be good friends by then.
Josephine 'Josie' Bethel: At first, Seth is hesitant around her, despite the innuendos and the jokes he throws in her direction. He believes that she reminds him too much of Alex, feels like he has too much baggage from his childhood and past, and there is also the fact that she is Apollo's daughter. He later begins to fall for Josie, realizing that he is in love with her in a way that he had never felt for Alex. He later sacrifices his love for her and becomes immortal because of it. They both become immortal and even end up getting married. They have a child named Leon that they introduce to Aiden and Alex, named after Apollo's alias back when Apollo disguised as a sentinel at the covenant.
"Alex, you're starting to worry me. Insult me... or something."
"Their sorry asses didn’t have a chance in Hades."
"Say that I'm not just the Apollyon."
“No one has ever said they loved me or were in love with me, and actually meant it. But you..."
“That is a precious gift that I . . . that I am not worthy of.”
“I reached down, turned the knob, and found it locked. Not a problem. The energy of the fire element rolled down my arm, and heat wrapped around my palm, searing the knob and melting the insides of the lock. Lock be gone. Ha.”
“If something happened to you again? I would burn this fucking world to the ground and then I would destroy Olympus.”
“I’m not leaving you. We’re in this together, Josie … If there is anything in this world, there is us.”