And with that, Batsans is off to fight crime. Commissioner Alphys, one of these days, you're going to have to nail his feet to the ground.
In Undertale, if you kill Papyrus, another monster will later note that a present that was addressed to him disappeared from under the communal gift tree. The implication is that his brother, Sans, had placed the present there, and removed it after Papyrus' death.
Papyrus has a collection of action figures in his room, and I thought it would be really cute - and, in this context, sad - if that final present was a pair of action figures: one of Papyrus, and the other of Undyne, his closest friend. I'm sure that Papyrus' massive ego would be duly flattered by the sight of his likeness in action figure form.
Action Figure Undyne's pose was inspired by the flavor text in her neutral/pacifist battle: "Undyne points heroically towards the sky."
I had a bit of trouble with the pose; it seemed to me that Undyne would want to point her finger as high and dramatically as possible, but the shoulder plates restricted the arm movement somewhat, so it took a few tries to get an angle I was happy with. Still, I like how her armor turned out.