Notes:

Original:

MFW it's been forever since I've been in the Undertale gallery:

Okay, so, like… I mentioned this over 5 months ago, and finished(?) it way back then. I've also got the entire recolor of Revy to be Undyne as well as a redone Undyne sprite to be Revy.
…I have no idea why I have such a hard time getting myself to actually upload the s--- I edit.
…there's also the Lala (MonMusu) battle thing I posted parts of and an Utsuho (Touhou) one I put days of work into to get part of it done…

Anyway, it's not really easy to pin things down like "this character is this character" due to varying conflicting reasons. Look at Alphys's face. She's panicking and confused trying to figure it out just as much as I have.

WARNING!! Long-winded overthinking below:
Like, Revy is Undyne. Done. However, Benny is the nerdy computer person operating behind the scenes who knows what they're doing is… morally questionable… which pretty much goes against their personal ideals, but it's not like they have much of a choice (probably inaccurate) – obviously that would be Alphys, right? I mean… Rock could work as Alphys, but he's a guy who is WAY too nice and harmless for the situations he gets in while Revy (Undyne) knows he's not cut out to be a member of Lagoon Company (royal guard)… that's Papyrus. So then we end up with… Undyne x Papyrus… [extra-private, one-on-one training intensifies]
OH WELL

Some ideas for character matching:
Dutch is Asgore because he's the calm, friendly, level-headed big guy who prefers to avoid violence, but knows how to put his morals aside when something must be done (for example, the child trafficking part with Garcia) – he can even keep Revy (Undyne) in check… which is something almost no one can do.
Balalaika is Toriel because… … well, because.
Chang would be Sans. He's on a skill level way above anyone, and he's super laid-back.
Sawyer is (box) Mettaton because robot voice and chainsaw.
Shenhua is Mettaton EX because legs and treats their employee (Leigharch) like s---.
Leigharch is Burgerpants because fucking hiiiigh
Hansel and Gretel are both Chara… friggin' androgynous demon psycho murder shitkids.

Of course, multiple characters from one thing fit multiple characters in the other, so there's a lot of… complications… and the types of associations between them vary greatly…