Esri Tuatha Dé is one of the secondary characters of the series. She is the mother of Lugh Tuatha Dé and also a noblewoman of House Viekone and is a unspecified relative of Dia.
Esri is an adult woman with long light purple hair and blue eyes. Despite being in the 40s, she can easily be taken for someone in her 20s, most likely due to a inherented ability from her family.
As Lugh's adoring mother, she has a huge son-con. She can be a bit of an airhead but is also clever in her own ways. She loves her son dearly much to the latter's chagrin. She likes to tease him as much as she can as well as Tarte whom she actively encouraged for them to be together. Cian told Lugh that it may be destined that men like them becomes weak in front of women they love, implying that Esri holds great authority over her husband. Lugh considers her insane, having made him - a boy - dress like a girl; she backs off after he turns 14, when he starts giving Esri stern reprimands.
Being his wife, she has been trained in the art of assassination and knows not to mix private and work life. She is supportive of her family's endeavours but becomes dispirited since she felt Lugh grew up too quickly. She can be extremely ignorant since she once tasked Tarte to seduce Lugh, both as an assassin and as his maid, telling her that women like them have to use their trump cards to be useful. This infuriated Lugh greatly who after reprimanding Tarte, went straight to his mother's room. She was still clueless as she felt that disappointed after knowing nothing happened and offering to teach them. She was petrified when Lugh showed his anger to her for the first time, warning her that he’d never speak with her again if she pulls the same trick. She apologised profusely but soon her antics made Lugh question whether she was really genuine.
Cian and Esri had a child before Lugh though he/she died at a young age.
When Lugh Tuatha Dé was born, Esri wonders if Cian Tuatha Dé will bring him into the clan and Cian confirms as the world needs Tuatha Dé. Esri accepts it although she isn't very happy about it, but Cian promises that he won't let Lugh die.
Much like Dia, due to the Viekone's blood, in her youth, Esri was targeted by an aristocrat whose goal was to procreate and have a powerful child with her. It was Cian, Lugh's father, who out of love and affection towards Esri kidnapped her to prevent that situation and later made her his wife with whom to this day he had two children and another one soon to come. 
Being her mother, she was initially against raising Lugh as an assassin of House Tuatha Dé especially after the incident with their first child, showing that she deeply cares for him. She was finally convinced by her husband who promised that he would protect Lugh with all of his power. She loves to tease her son but is sadden at the fact that he grows up faster than she wished. Lugh is constantly annoyed at his mother antics such as when she stayed up to check whether he had wet dreams, and when she fills up Tarte's mind with needless information. That said, he still cares immensely for her mother who he knows has his best interest at heart.
- Her name is inspired by Eithle (Old Orish: Ethniu), wife of physician Cian and mother of warrior king Lugh of the Tuathe Dé (tribe of gods) in Irish mythology.
- volume 6, chapter 8