The Mystery Cartoon Character Game, ongoing.

Got it Glowworm. His full name is I.M.A. Sureshot.

He is an exterminator who Porky hired to get rid of Pierre the termite.

You're up!

Yeah, I had to look up his name--haven't seen the short in a while, but I used to see it a lot and the clue about his plunger was the clincher for me.

Anyhow,
1. My short is a Canadian one.
2. Set to a jaunty tune, my tale is sung and told by a narrator.
3. I am not popular or handsome.

Sorry, but I'm not very keen on Canadian toons.

Here are some more clues.
4. Fate takes a hand, when one day I gain an abnormal feature on my head that resembles a human face.
5. The face is handsomer than my own, so I pass it off as mine by hiding my real one.
6. Sure enough, I gain instant popularity.

I'll really have to look this one up.

Found it! It's George from the short, The Lump.

Yes, It's George from the 1991 short, "The Lump"

1. The Lump is a National Film Board of Canada animation.
2. Quite obvious.
3. George was really ugly and short and no one would pay attention to him.
4. One day, George gets a lump on his head.
5. Quite obvious, George is uncomfortable walking around in public looking like he has two heads, so he covers up his own and passes the head off as his.
6. Suddenly, everyone loves George!

OK, here it goes.

I appear once in an animated movie.
It's just me and my sidekick.
Since my sidekick is an animal, I talk for him.

Again, I thought I had it, but I don't.

Well, not an animated movie, more like a cartoon short film, but this sounds like Larry from Seth MacFarlane's pilot short Larry And Steve, which would ultimately morph into Family Guy. To everyone else, Steve's attempts at speech are heard only as dog barking; Larry, being a supreme idiot, understands Steve perfectly.

Nope, not Larry. The character in question is in a full length movie.

Three more.

I fall for the protagonist in the story.
However, I am not her first choice.
I've been harvesting/ selling since I was a kid.

Sorry, but I still don't have it.

I knew it--just had to make certain. It's Kristoff from Frozen.

That's right.

He appeared in Frozen.
It's just him and his reindeer, Sven.
He talks for Sven in a reindeer type voice.
He fell for Anna.
She was already engaged.
He harvest/ sells ice.

Yeah, the minute you mentioned the protagonist talked for his animal sidekick--I knew it had to be Kristoff--however, the way you mentioned that it was just him and his sidekick made it sound like it was a movie that only revolved around two characters instead of many.

Let's go with something easy today.

1. I fall for a human.
2. The original story my movie is based on, has a tragic ending.
3. However, I live in this version.

Ariel, from The Little Mermaid.

Yeah, simple and to the point. I highly doubt I need to explain the clues for this one--but here it goes.

1. Ariel falls for Prince Eric.
2. In the Hans Christian Anderson tale--the nameless mermaid fails to gain the love of the prince, and dies.
3. Of course, since this is a Disney movie, Ariel gains Eric's love, gets to be a human, and marries him.

Alright, I've been contemplating on using this character and decided to go for it.

I appear in a segment of a movie.
The whole segment is unsettling.
I first reveled myself as friendly (in a creepy way).

I'm going to speculate, rather than brick-throw, that this is Elahrairah, the Prince of Rabbits from Nepenthe Productions adaptation of Richard Adams's best-seller Watership Down. Elahrairah appears to Fiver, after some other rabbits tell him that his brother was shot while attempting a rescue of hutch rabbits at the Nuthanger farm. Fiver follows the ghostly specter of Elahrairah, certain that he'll lead Fiver to his hurt brother Hazel. However, according to the book, Fiver regards this as ominous, since Fiver has the ability to glimpse the future, and Elahrairah surely will exact a horrific price for this. Fortunately, Elahrairah isn't interested in Fiver's anguish, but instead in seeing that the clever and resourceful Hazel recover his health.