I think Undyne and Asriel were friends. Allow me to explain.

so undyne hates humans quite a bit. Obviously humans haven't been great to monsters, but she hates humans more than anyone. Papyrus? Just imitating her, trying to be popular. Alphys and Mettaton LIKE humans. ASGORE isn't doing what he does out of hate. Even monsters who try harder than most to kill you apologize or see the error of their ways, but Undyne has to be painstakingly befriended before she realizes what she did wrong. Unlike everyone else, she's EXCITED to kill a human. Almost like she has a personal vendetta against humans.

During the undyne hangout, when you spare her while her house is burning down, she calls you a "wimpy loser with a big heart... just like him." Who is this Him? she looks crestfallen at the mention of 'Him', as if this Him is gone or something happened to Him. She speaks of 'Him' similarly to how ASGORE speaks of his children.

Undyne and ASGORE are also clearly very close. ASGORE trained her as a kid, after all. Kid Undyne probably spent a lot of time around ASGORE, which, if Asriel was alive at the time, would've meant Asriel and her saw each other a good bit.

Another thing - Undyne knows EXACTLY what a human looks like. plenty of monsters never realize what you are, (Monster Kid, for example) some have to ask ("SANS... IS THAT A... HUMAN???") some take awhile to put two and two together (Royal Guards 1 and 2) and most monsters just seem generally confused. Some of the few people who are confident in what you are are the Dreemurs and Undyne.

It stands to reason that the Dreemurs recognize humans, but Undyne less so. Unless, that is, she was friends with Asriel, she would have seen Chara plenty.

Anyway, thanks for coming to my ted talk. What do you guys think?