'Sans C. Aster is a character from RTNightmare's CounterTale, who was originally only the Judge of Monsters. However, after King Asgore and Queen were kidnapped by humans, Sans was chosen to be king as the most eligible - Undyne being the second choice. Since then, he's harbored a greater hatred against humans because of all the work he was forced to take on.
When Sans first met Rachel, he was very judgmental of her because of her human appearance. But after she saves Papyrus, the missing/kidnapped monsters that were held prisoner by the evil Human Empire council, and helped him in so many ways, he warmed up to her and eventually, fell in love.
By Arc 4, he is married to her, has had at least one child, and is running the kingdom with her help. His attire changes later to befit him.