Maria di Angelo

Maria Di Angelo was an Italian-Amarican woman and is the mother of Bianca and Nico di Angelo. She could see through the Mist, which caused Hades to fall in love with her.

The Demigod Files
In Percy Jackson and the Sword of Hades, Nico's mother visits him disguised as the goddess of ghosts, Melinoe. She is described by Percy as wearing a black dress with white gloves and pearls, with her dark hair pulled back. He couldn't make out her face, but in The Last Olympian, it is said she has Bianca's smile and Nico's eyes.

The Last Olympian
When trying to summon Maria, Nico di Angelo instead finds much information about her:

During the time she and Hades were together, the Great Prophecy had been spoken and World War II was beginning with some of his other demigod children involved. He wanted to hide Maria and the children in the Underworld so that the other gods wouldn't find out about them. Maria refused to be hidden or separated from him or the children saying they needed her and was openly unafraid of Zeus despite the fact Hades darkly warned her that she didn't know his family.

While they were in a hotel, Zeus destroyed it, killing Maria, and Hades was only quick enough to shield Bianca and Nico. Hades had wished to revive her in his loss but Alecto the Fury reminded him that as an immortal he had to respect the forces of life and death so Hades ordered Alecto to take her to the Underworld to be given her peace.

Hades love her, as Annabeth pointed out the gods rarely reveal their true nature to a mortal and because he had two children with her (unlike having twins) and continued to see her until Bianca and Nico were twelve and ten. She was also able to see more to him than the dark person everyone else saw and he put up saying if the other gods could see him like she did perhaps they wouldn't fear and outcast him. Her death caused Hades a lot of pain and bitterness towards his family which led to the curse on the Oracle and the first death of Thalia. Nico was able to partially use the memory of his mother to convince Hades to fight with the others telling him she was right about families sticking together and he should prove himself to be more that what his siblings thought of him.