Ah, the Whos. The Grinch's clueless enemies, who seem to have no idea that their odd neighbor to the north is plotting obsessively to destroy their spirits.
The Whos aren't exactly individuals. Cindy-Lou Who's the only one who ever gets a name. So we're gonna go ahead and take them as one big group. Without them, it would simply be a book about the Grinch muttering under his breath and being irate about… nothing at all. After all, the Grinch is miserable, and misery loves company.
If you took the Grinch's word (which you, um, should not), the Whos are just a bunch of loud and obnoxious neighbors:
[…] All the Who girls and boys
Would wake bright and early. They'd rush for their toys!
And then! Oh, the noise! Oh, the Noise! Noise! Noise! Noise!
That's one thing he hated! The NOISE! NOISE! NOISE! NOISE! (27-30)
The Grinch is irritated by the Whos' celebration and how much noise they make when they are having fun. Most of all, he's irritated that they are having such a raucous good time on Christmas morning; who are they to deserve that kind of merriment, anyway?
The Grinch's entire existence revolves around his desire to bring the Whos down to his own miserable level. So you might say that the Whos as a whole serve as a projection of the Grinch's own loneliness and his jealousy of those who have a community to turn to. They're everything he's not because they have each other, and he has… his dog.
As a collective, the Whos prevail over the Grinch, even though they are smaller and seemingly at a disadvantage because he's, you know, burgled all of their homes.
And how do they prevail? By sticking together, of course. As they join in hands, they manage to show the Grinch that they are stronger than he realizes. It's more than just shiny ribbons and pot roasts that bring these folks together:
Every Who down in Who-ville, the tall and the small,
Was singing! Without any presents at all! (157-158)
With their strength in numbers and mad singing skills, they manage to prove that Christmas was never lost at all.