Semele was the demigod daughter of Harmonia, a princess and the mother of the god Dionysus. After her son ascended to godhood, she was revived from the Underworld and made a Goddess, Thyone.
History
Dionysus, her son.
Zeus noticed Semele one day and the two entered a relationship. When Hera found out she went to her and made her doubt Zeus was who he said he was and to have him swear on Styx to give her anything she wanted. The next time they met Semele, who was pregnant, had Zeus swear an oath on the Styx to show her his Divine Form, he tried to talk her out of it to no success but she was insistent. He reluctantly did so and was killed. However Zeus managed to save their unborn child and sew him into his thigh until he was old enough. Later, Dionysus saves Semele from the Underworld, and makes her immortal.
Due to being mostly forgotten, Semele began fading and became an eidolon, forgetting who she really was. After the Quest for Iapetus, she was one of several mythics who sought to change their ways.
The Nico di Angelo Adventures
The Court of the Dead
Shortly after resettling in Camp Jupiter, Hazel Levesque brings Nico di Angelo and Will Solace to the mythic's barracks to introduce the boys to the refugees. Semele becomes upset at the accusation that she will possess someone and rebuffs that she will never do so without permission. Upon hearing their talking to them, Nico and Will agree to help get the Twelfth Legion Fulminata to accept them.
Later that day, She and the other mythics go to the Mess Hall for dinner, causing a tense atmosphere with the legion. When they leave, the legionnaires lighten up and talk loudly the second they are out of sight. The following day, she accidentally phases through a girl and tries to apologize. However, earlier events have created a tense atmosphere and other legionnaires start to gather around them. Nico gets her out and she thinks Arielle is right about leaving. When the son of Hades asks about the group being hunted, she tells him to ask Johan.
After Arielle vanishes, she contacts Hazel and, after the praetor brings Nico and Will with her, they debate on staying put or not for their own safety. When Quinoa goes missing, the legion organizes a stakeout to catch the perpetrator, however she is skeptical of the idea. Despite their best efforts, Asterion is taken. Semele takes charge and tells Orcus and Johan they will leave in the morning. Hazel tries to talk them out of it and asks Nico if he sensed anything out of the ordinary. The son of Hades states he felt something shortly before the others as he realizes he was in contact with one of the cacodemons. When Semele questions how the perpetrator could get passed Hazel, the girl explains that the Mist is meant to trick the minds of mortals and that, due to their mix of divine and mortal heritage, they can see through the Mist to an extant, but can still be tricked by it. As the perpetrator could see through Hazel's illusion, she thinks they are dealing with a minor god. After realizing Hazel is the next target, Frank suggests they hunker down in their vault and Hazel invites the mythics and asks Nico to lend her a cacodemon. He agrees and the praetors decide to focus security on a single area for better chances at success. After persuasion, the mythics agree to stay one more night and everyone turns in.
By morning, everyone at Camp Jupiter begins to take the disappearances seriously after learning that Hazel is a target as well. A few even pledge to keep the remaining mythics safe. Semele, Johan, and Orcus join Nico, Will, and the praetors to finalize the plan. Nico, Will, Hazel and the mythics will spend the night in the vault as Frank and a few legionnaires stand guard outside. All participants will have a cacodemon with them to serve as an early warning signal. Once night falls, the plan goes into motion. The mythics explore the vault as the demigods get the sleeping bags ready. Everyone is nervous, but focuses on the task at hand. At 3:00AM, everyone is awake and the cacodemons go into a panic. Nico and Hazel draw their swords and Hazel focuses on something that only she can see. Soon, the daughter of Pluto pins someone to the wall and a disheveled woman becomes visible. After Hazel draws her blood, the group realizes she is a goddess. Everyone demands to know what the goddess did with the mythics, however she only states that she has brought them to the judged and sentenced before demanding Hazel come with her. The woman unleashes a blast of force that sends Will, Johan, and Orcus flying into the wall. Semele checks on them as the goddess tries to take Hazel, but is driven off by Terminus, Will and the Coco Puffs. Everyone wonders what the officer's badge means as they go to check on the sentries. Once the group meets up with Frank, Hazel decides to go with Nico and Will to the Courthouse while the son of Mars and the mythics stay behind to safeguard the camp.
After the mission falls and Asterion and Hazel are believed to have been killed, she speaks with Nico in the Mess Hall. She informs him that there are no signs of Asterion regenerated in their quarters. Semele then talks about how the eidolon were forced to possess others for the goals of others and that each time, they lost a part of themselves. Semele reveals she remembers fragments of her life: born into royalty, an encounter with Zeus, and having a son. She then tells Nico that the remaining mythics want to help however they can. She leaves as Will and Frank arrive. After the son of Mars organizes a rescue party, she joins them. After Will's mother arrives at Camp with news of Hazel's whereabouts, she greets the singer and listens as Johan explains why Pirithous was punished in the first place.
After arriving at Golden Gate Park, she possesses Nico after the son of Hades is greatly weakened saving the rescue party when the Mist breaks. After Nico reunites with Hazel, the daughter of Pluto fills them in. When the siblings fail to free Apate and Dolus, she suggests a council can release the gods and the cacodemons gnaw at their binds. Semele reveals her identity to the tricksters to get them to cooperate and, when they are freed, they restore the Mist. However, everyone present is placed in a personal illusion. She aids Nico in finding the twins best as she can. However, she begins to inadvertently take control of him. After convincing the twins to focus only on Pirithous, the two take her, Nico, and Will to the judge after Hazel and Quinoa incapacitate him and take out his guards. Everyone gathers and, after Dolus and Apate convince Pirithous to release his followers, everyone discusses what to do with the judge as Hecate arrives. Everyone bows to the goddess of magic as Pirithous explains his actions to Hecate. Everyone stays quiet as the judge crawls to the rock and is fused with it before being sent to the Fields of Asphodel. Hecate notices her in Nico and extracts Semele from the son of Hades. Being close, Hecate returns her to her physical form. Semele then thanks her allies before going to see her son Dionysus at Camp Half-Blood. They spend the night catching up.
Later in one of her letters to Nico and Will, Semele was revealed to have gone on a road trip with her son and Argus to Florida after convincing the Olympian Council to let Dionysus go with her. Upon arriving in Florida, the trio set up a beach house with some help from Xenia, the goddess of hospitality and went into wild night of partying including partying with a biker gang from Louisville.