Hades met Maria in a graveyard in Italy.
He rarely leaves the Underworld, especially if it isn't business forcing him to do so, but on very rare occasions, he likes a change of scenery. So he'll go up to the mortal world with a book or something and find somewhere nice and quiet to relax for a bit. He tends to gravitate towards graveyards and other places where the dead are kept for obvious reasons. And one time, the graveyard he chose happened to be one in Italy, and a woman named Maria di Angelo happened to be visiting.
Because Hades was relying on the Mist to hide him from mortal eyes, he hadn't bothered to change his appearance. But Maria could see through the mist almost as easily as gods can. And she saw a very tall man, built like a linebacker with so many scars, long, electric blue hair, and yellow eyes, wearing all black with makeup, jewlery, and nail polish. It was a very bold look for 1920s Italy and she wanted to get to know him better. So, she went up to him and asked if she could sit nearby. He said yes, and the two of them just sat there for hours, reading and never saying a word. When she was done with her book, she rose and asked if they could meet again, maybe same time this day next week? Out of curiousity, he said yes.
This mortal woman who saw him through the Mist and yet approached him without fear and offered him friendship intrigued him. So he went down to the Underworld and told Persephone about her. Persephone was thrilled Hades was making a friend, and once he was more comfortable with Maria, Seph wanted to meet her.
Hades and Maria's weekly graveyard book clubs went on for nearly a year before they started having actual conversations. And Hades? Oh, he fell hard and fast (quite literally. Getting hit with Cupid's arrow is not a pleasant experience for gods, it knocked him over.) He discussed it first with Persephone- he would never have acted on it without her consent- and she decided that she had no problem sharing him, but she wanted to meet this woman. So Hades bought her to the next book club. Maria's hot crush bought with him an ever hotter woman who he said was his wife, and now she had a crush on both of them. The concept of nonmonagamy was a bit unheard of for Maria, but she took to the idea rather well. She and Hades started seeing each other. And while originally she was just friends with Persephone, that eventually evolved into a romance as well. Of course, Hades is the only one her family was aware of (and they thought he was a high-ranking member of the mafia,) but she was very much with both. They both absolutely adored her.