There are likely some out there. You can start by checking fanfiction websites like Archive of Our Own. Just search for 'older Judith x Negan' in their search bar, and you might find a variety of stories with different plotlines and interpretations of their relationship.
One possible theme could be redemption. Given Negan's past actions, a story could explore how he tries to make amends in Judith's eyes as she gets older. Another theme might be mentorship, with Negan imparting his survival skills or knowledge to Judith.
Judith in the Judith Bible story is a heroic and devout widow. She was a woman of great courage and faith. She played a crucial role in saving her people from the Assyrian threat.
I don't have enough information to determine exactly who Judith Kay is. She could be an ordinary woman who gets caught up in an extraordinary situation involving the mirror.
Well, Negan first appears in the series as a major antagonist. He has a very distinct personality. He's confident, often to the point of being cocky. His group, the Saviors, would extort other communities for resources. He uses fear as a tool to control those around him. His actions have a huge impact on the other characters in the show, like Rick and his group.