Some movies have heros like John Wayne who look like every other person, only act differently. We definitely remember them, but we might remember the actor more than the characters because the characters tend to blend over time and not leave a lasting impression. Everyone knows that John Wayne was a famous actor, not many people could actually name many characters he played. We do however recognize the icon of "cowboy." The hat would give it away even if he was in silhouette.
Other movies have iconic heroes who not only act differently - but they look different. Usually these would be super-hero movies or science fiction fantasy films. Not only do these types of characters seem to be more inclined to launch merchandising campaigns, but they seem much more inclined to ingratiate themselves into pop culture and last indefinitely.
When designers are inventing logos - one of the primary tasks they concern themselves with is the silhouette - the shape of the logo. Whether the logo is a symbol like Mercedes or Apple or a word like Levis or SEGA - in the end both become iconic representations of the company which are designed to leave a lasting impression in viewer's minds.
In the late 20's Walt Disney created a Character called "Oswald the Lucky Rabbit". He was a bunny who had long rabbit ears and a short body. Pretty expected.
He followed up this character with a character who looked remarkably similar except it was a mouse with huge iconic ears. It breaks expectation.
Today, not only do we still remember Mickey Mouse, but the entire Disney brand is identified by this single iconic element.
Big isn't always the key though - All mice have ears, making his ears big just set him apart by breaking expectation. Not unlike Dumbo - another long lasting character.
But if you put ears on human characters - you are also breaking expectation.
Loveless incorporated ears and tails as a indication of youth and virginity to delineate the children from the adults. But Loveless is not the only anime to use the cat features on humans - in fact, there is even a name for the phenomenon - Nekojin and there are way too many to list.
The impression it leaves on the fans is easy to see at conventions.
There are another commonly seen set of ears worn by people who are not always even fans of the source. Can you think of it? What is the most common high school girl halloween outfit? Bunny ears. What does it indicate? Playboy - an adult magazine feature nude photos. Much like Disney - the ears became a symbol of the entire company.
They break the expected silouette.
Who leaves a bigger impresion - Luke Skywalker or Princess Leia?
The side buns spawned a great deal of imitation and even when in subsequent episodes Carrie Fisher was nearly undressed in her costumes - nothing quite left the same impression as her initial outfit because of it's unique silhouette change. It is imitated even stil today 30 years later and everyone understands the side buns means Princess Leia no matter who wears them.
Beyond individuals though - entire group silhouettes can have the same affect... The Wizard of Oz is a perfect example.
Just like with the apple and mercedes logos - you could darken the characters of this Wizard of Oz poster and everyone would still know exactly who these characters are and what movie this is... even though the movie was made in the first half of last century.
People like to recognize. It makes them feel like they belong.
There is no guarantee that any character design will create a lasting iconic effect on an audience - but by borrowing a page from the logo designer's handbook and taking into consideration the silhouette of the characters, you will definitely be giving yourself an edge towards creating a character which breaks expectation and therefore might create a stronger impression on your audience.