Yukariko Sanzenin (三千院 紫子,Sanzen'in Yukariko?) is Nagi's deceased mother. She appears in Nagi's dream and Hayate's illusion. She did not marry into the Sanzenin family, but rather she is the daughter of Mikado Sanzenin. She died of sickness when Nagi was about 5 years old. She often said to Nagi that she would always look after her as the sky and the stars. In the Anime, she made her first apperance in Hayate no Gotoku! Episode 24.
According to Hatsuho, Isumi's mother, her appearance resembles Hayate's, which is why Nagi is so fond of him. When she was younger she was already ill and was being treated for some disease. She was a kind and caring woman, despite being outsmarted by her daughter.
Yukariko was friends with Mikoto Tachibana (apparently Mikoto and Yukariko were childhood friends) and Hatsuho Saginomiya (Isumi referred to Yukariko as the senior that her mother loved her as an older sister), both of them loved her very much. When Mikoto and Wataru were gambling in Las Vegas, she said that Yukariko was loved by the gods so she was the luckiest person in the world until she became sick, she also remembers that Yukariko married the man she loved, showing in her memories a young Yukariko with a man in a butler suit whose hairstyle was similar to Hayate's.
It is also seen in chapter 187, that Yukariko was helped by Hayate Ayasaki's brother, Ikusa Ayasaki
In some flashbacks Yukariko as a child is seen playing around the mansion and finding what looked like King's Jewel. It is implied that was how she entered the Royal Garden and gained the "Power of Royalty" since she was the "chosen one by God". It is revealed by Klaus that Yukariko once held a crush on him as a child as she once asked to marry him and gave him an apartment as a gift. Her grave is at Shimoda, and Nagi visits it every year.