He was Marta's vampire lover, and the father of her son, Lindo. Marta was shown to regret their relationship after Lindo was born, thinking that her son had been devoured by a vampire, presumably Nesta.
Nesta himself claims that he never loved Marta.
Lindo is Nesta's Dhampir son. It is unknown if Lindo knows about his father, but Nesta clearly knows about him, saying that his life is in the palm of his hand and always has been. In episode 11, he is curious about his son, at first, asking if he looked like him, and if he had yet fed on human blood. However, he expresses disgust and disappointment when he learns that Lindo does not share his gluttonous bloodlust, saying that even a half-vampire is still only half, and that he should've known Lindo would be a disappointment.
Nesta, to his downfall, underestimated Lindo's will to protect Ritsuka and overestimated his son's weakness. He thought that he could make Lindo side with him by telling lies that he truly loved Marta and wanted to be with his family as well as preying on Lindo's lack of bonds with his biological family, his fatherless life, and his mother's rejection of him. Lindo is stronger than he looks, and his strong emotions helped him to break free from Nesta's powers at the last moment.
Nesta is Jek's master, whom he serves faithfully.
She is his niece, but like Lindo, the two have no love for each other. Ritsuka is disgusted with Nesta's behavior and upset to realize that the vampires were lying to her all along- Nesta was planning to kill her to get the grimoire instead of letting her live as Jek initially proclaimed. Once she realizes his plans, Ritsuka attempts to escape from him and prevent his plans from progressing. She was enraged when Nesta attempted to kill her cousin, her devil allies, and her true love, Rem. Her desire to protect Rem and her friends and family serves as a catalyst to awakening the grimoire's powers within her, and she uses the grimoire to find Nesta's real heart and kill him once and for all.
Despite her disgust and dislike for him, Ritsuka pities him as a vampire that cannot go back to being a human and die in peace.