Mayu is said to have served many gods, two of which are Yato and Tenjin, her current master. Prior to the series’ start, she was working with Yato under the name Tomo. She worked with Yato for three months, and when the series started it was the last day of her job as Yato’s shinki.
In Chapter 43, Mayu's past has been revealed. She had a husband, who was in the army, and a daughter named Kiyoko. She is killed by fire on a burning bridge during a war occurring in Japan (Presumably WWII). She threw her daughter off the bridge to save her. This would, therefore, be a God's Greatest Secret.
Chapter 1: The Man In the Sweatsuit Mayu appeared together with Yato under the name Tomone in a girls’ toilet in Mutsumi’s school after they received a call from Mutsumi, who was hiding in the toilet. She was surprised to know that Yato had been doodling advertisements even in the school’s toilets, and when Yato introduced himself as a god, she told her customer that he was merely a lesser god. She was upset when Yato said that he could eliminate the people who bullied Mutsumi, and told him to be polite to his customers when Yato said that Mutsumi was arrogant and annoying.
When a large stingray-like phantom appeared, looming all over the school and began voicing the students’ opinions of Mutsumi, Tomone figured that slaying the phantom would end the bullying done on the girl. Yato called Tomone’s weapon form Hanki, and she transformed into a dagger and sliced the phantom, exterminating it. The moment the mission was finished, Tomone asked Yato to return her back to her human form as she complained about his sweaty hands, much to Yato’s dismay. She thought that Mutsumi would be all right, however, her guesses were wrong as slaying the phantom did not stop the bullying. Tomone tried to cheer Mutsumi up, but Mutsumi told her to shut up and leave, pissing Yato off as he kicked the infirmary screen and told Tomone to leave the girl.
Tomone was dissatisfied with Yato’s reaction and scolded him for leaving Mutsumi, forcing Yato to return to the school to sever the bonds between Mutsumi and her classmates using Hanki. Tomone became angry with his decision, saying that having to start anew was too cruel. Yato said that he didn’t want the gods to be treated as mere services, but Tomone countered that it was precisely because of his attitudes that they got no “customers” (worshippers). She demanded to quit being Yato’s shinki and cried loudly, forcing Yato to reluctantly release her name as Tomo. The moment her name was released, she immediately badmouthed Yato and left gleefully.
Chapter 2: The House Cat, The Stray God, and The Tail Mayu (as Tomone) was seen massaging Yato’s shoulders in his dreams of becoming a well-known god; although later she changed her expressions from obedient to horrified, as she complained that she didn’t want to serve under a lesser-known god with poor lifestyle, waking Yato from his dreams.
Chapter 4: A Few Godly Perspectives Mayu was called by Tenjin to help Yato for the mission to exterminate phantoms that caused at least two people to commit suicide on a railway crossing. Yato was shocked to see her again and called her Tomone, before she corrected her name as Mayu. As she brought the three to the location, Mayu told Hiyori and Yukine that she used to serve Yato, and told them that Yato was the worst master she ever had. Mayu and Yato then started bickering in front of the kids, making them wonder whether they were ex-lovers due to their bad relationship.
She showed them the location of the suicide and told the story behind it, and asked Yato to exterminate the phantom when Yato said that they should let those people with suicidal tendencies die, as they could no longer be saved. Mayu became angry and said that Yato was as vile as ever, and forced Yato to complete the mission as Tenjin had paid him in advance. She returned to her place afterward, without any mention of whether she knows of the mission completion.
Chapter 9: Abominable Creature Hiyori, Tenjin, Tsuyu, and Mayu talked about Strays on New Year’s Eve when Hiyori visited Tenjin’s shrine to pray for Yato and Yukine’s safety. However, Mayu and Tsuyu told Hiyori not to press the issue further, especially about Nora, showing their detestation towards her.
Chapter 11: A Name When Daikoku informed Tenjin of Yato’s grave situation due to the blight he received from Yukine’s misdeeds, Mayu immediately volunteered to help in the purification ritual as she was once Yato’s shinki. Together with Daikoku and Kazuma, they performed the purification ritual, and although at one point Yukine nearly changed into a phantom, with Hiyori’s intervention they were able to successfully cleanse both Yukine and Yato’s impurities. She scolded Yato for delaying the punishments as she hated playing doctor, and was relieved seeing Hiyori hugging both Yato and Yukine.
Chapter 12: To Cut, or Not To Cut Mayu talked with Yukine while her master was bickering with Yato. She told Yukine and Hiyori that one of Tenjin’s shinki Miyu had her name revoked and was banished as a result of cutting her wrist, influenced by the story of Yukine’s purification. She told Yukine that even though he committed serious offenses as a shinki, Yato still intended to keep him. She also told Yukine to always maintain the line between good and bad and to protect the name Yuki that was bestowed upon him by always obeying his master, to which Yukine vowed to do as told.
Chapter 18: Drawn Swords Mayu was seen worried for Hiyori and Yato after she heard of Hiyori’s kidnapping and Yato’s intentions to go to the heavens and save Hiyori. She felt that it would be a stormy night due to the negative aura gathering in the sky.
Chapter 22: What Must Be Done Upon the return of Yato, Yukine, and Hiyori back to Tenjin’s main shrine, Mayu cried happily for their safe return. She even hugged Yukine, thinking that he must be scared after all the fighting. She was surprised to know that Yukine had become a blessed regalia.
Chapter 43: Secrets While Yato performs an unusual request, it is revealed that Mayu was the mother of a young girl named Kiyoko during the war (presumably WWII). Her husband had been drafted into the military, leaving her to take care of her daughter alone. During what appears to be the firebombing of Tokyo, she and her child became trapped in a fire, but she saved her child by throwing her into the canal before she herself perished in the fire. Her last words were that her daughter should try to live for her. She has no memory of this event at the time it is revealed.
Episode 1: A Housecat, A Stray God and A Tail Mayu appeared alongside Yato under the name Tomone on the top of a school building, in a mission to save a middle schooler named Mutsumi who was bullied by her classmates. She was shocked to see a large stingray-like phantom was looming all over the school when Yato told her that it was due to the examination season when the students were mostly on edge.
Yato called Tomone’s weapon form Hanki and used the dagger to slay the phantom using the Rend skill, and complained about his sweaty hands. She told Yato that she wanted to quit working as his shinki, and Yato tried to stop her before she cried out loud saying that she had enough of living like a vagrant with him. This caused Yato to reluctantly release Tomone’s name, and upon her name’s removal, she began badmouthing Yato and left gleefully, much to Yato's anger.
Episode 3: Bidden Calamity Mayu was called by Tenjin to help Yato for the mission to exterminate phantoms that caused at least two people to commit suicide on a railway crossing. Yato was shocked to see her again and called her Tomone despite her protests. Mayu and Yato began bickering in front of Yukine and Hiyori, making them wonder whether they were ex-lovers due to their bad relationship, to which Mayu and Yato denied. Mayu told Yukine and Hiyori that she used to work for Yato and told them that he is a terrible master.
She showed them the location of the suicide and asked Yato to exterminate the phantom when Yato said that they should let those people with suicidal tendencies die, as they could no longer be saved. Mayu became angry and said that Yato was as vile as ever, and forced Yato to complete the mission as Tenjin had paid him in advance. After the mission was completed, Mayu bowed in gratitude towards Yato and left the railway crossing.
Episode 7: Uncertainty, Destiny During Hiyori’s search for Yukine at Tenjin’s shrine, Tenjin, Mayu, Tsuyu, and Hiyori talked about Strays. However, Mayu and Tsuyu told Hiyori not to press the issue further, especially about Nora, as they showed disgust towards them.
Episode 9: Name When Daikoku visited Tenjin’s shrine in search of a shinki to help with Yato’s purification, Mayu volunteered to help out, saying that she was once Yato’s shinki. She was shocked to see Yato’s serious state and tried calling out to Yato. When Yato refused to revoke Yukine’s name as asked by Kazuma, Mayu said that Yato didn’t want to listen to them, implying that he insisted to have them do the purification.
Together with Daikoku and Kazuma, Mayu performed the purification ritual towards Yukine. Although Yukine’s blight was so severe that he nearly changed into a phantom, Mayu held on and with Hiyori’s help, the ritual ended with a success. Mayu was happy to see that Yato and Yukine were both cleansed from the blight although she was exhausted after performing the ritual.
Episode 10: Regarded with Hate Mayu talked with Yukine while her master was bickering with Yato. She told Yukine and Hiyori that one of Tenjin’s shinki Miyu had her name revoked and was banished as a result of cutting her wrist, influenced by the story of Yukine’s purification. She told Yukine that even though he committed serious offenses as a shinki, Yato still intended to keep him. She also told Yukine to always maintain the line between good and bad and to protect the name Yuki that was bestowed upon him by always obeying his master, to which Yukine vowed to do as told.
Episode 11: Abandoned God Mayu was seen when Phantoms and Gloom started appearing at an alarming rate. Mayu later stood next to Tenjin as Yato told him about Rabo, to which Tenjin then commented on Rabō once being Human.
Episode 12: Scraps of a Memory Mayu was seen watching as the sky was filled with a large Storm, and became worried about Yato.
Mayu appeared when Yukine was watching Yato divine-possessing Hiyori and caused mischief, and told the boy to follow her to get help from Tenjin as the incident was out of a shinki’s capability. She and Yukine told Tenjin of Yato’s mischief, and together she and Tenjin went to Hiyori’s high school dressed as noble-looking people. However, their appearances attracted the schoolgirls and things quickly escalated as the girls asked for love advice.
Despite the obstacle they faced, Mayu and Tenjin managed to capture Yato together with Yukine and Hiyori.