(My AU - DON'T COMPLAIN!!!)
One time, in Alphys's lab, they was an earthquake and Undyne save Alphys but got a head injury that left her in a coma for seven months. Alphys blames herself for letting Undyne get hurt , so in order to protect Undyne, she broke up with her.
Heart broken, Undyne refused to fight or talk to anyone. Papyrus, whom has became a royal guard, goes to talk to her and asks her if she was available for a simple dinner.
When they were at dinner, Undyne cheered up a bit. So each weekend, they went to dinner and eventually their first date. Days turned into weeks, weeks turned into months and months turned into years.
Two years after they became couple, Papyrus asks Undyne to marry him, of course, she said yes.
About a month or two later they got married. It was a celebration for everyone: their heroine has return to her normal state and Papyrus could never be happier. Even Sans felt joy to his little brother's wedding. Alphys attended the wedding as well, making Undyne happy that she came to her wedding.
About ten months past, and a little skeleton and fish hybrid was born into the world.