Aliza herself. While she lacks the sheer determination of Frisk, she more than makes up for it with perseverance and ingenuity. Especially since after all of the absolutely horrible ways she's died, she's still managed to stay sane.
Dogaressa stands up to Undyne and her guards after a string of events lead to a disguised Aliza (who, due to Papyrus messing up an attempted alibi, everyone thinks is his niece which, therefore, makes her Sans' daughter) being put in Undyne's crosshairs. One guard asks her if she thinks she'll do better than her 'dearly departed husband', which pushes her Berserk Button hard enough for her to kill him in one strike. It's heavily implied she would have started on the rest had Undyne not taken Papyrus hostage.
Dogaressa: Yes. I do.
Sans' fight against Undyne and her guards once they try to kill him. After it becomes apparent they won't listen to reason, the supposed slacker fights back and absolutely floors them with his usage of bones, Gaster Blasters, etc. The fight culminates in Sans outright catching Undyne's own spear with his magic and throwing it against her face, destroying it beyond repair. Sadly, this tires him out long enough for Undyne to pull his eye out and extract his magic. On that note...
While Sans destroying the CORE is by no means a good thing (in fact, it's what led to everything being fucked up beyond repair), but it's also an awe-inspiring display of just how unstoppably powerful he is. It's telling that Undyne hasn't attempted to have him assassinated despite what he did; she may very well be terrified of him.
The Royal Guards aren't afraid to fight the entire civilization if Undyne commands it. The second Sans shows up, however, they all freeze on sight and remain speechless.
