How do I befriend Undyne?

So, I just beat undertale, and I got the neutral ending. At the end,

Flowey told me that I wasn't a good enough friend to Papyrus and Undyne. Then I talked to Papyrus, and he said Undyne wants to beat me up.

Now, I have the same save and all of my progress. I already went on a date with Papyrus, but now I can't find out how to befriend Undyne. I thought that I had already befriended Undyne, since after fighting her:

I poured water on her armor so that she wouldn't die.

Apparently that isn't enough. What do I need to do next?

Solution 1:

In order to truly befriend Undyne, you need to "date" Papyrus and befriend him, then go to Undyne's house in Waterfall (near Napstablook's house right after the trash zone). Talk to Papyrus to initiate the hangout with Undyne.

You also have to have not killed a single other monster during your playthrough. If you have, Undyne will refuse to do anything with you.