The bBest Gaijin Character?

His machine is called Caesar Caesar Emperor (シーザー・カエサル・エンペラー), and its skill is “Sentimental Outlaw Blues.”

This is really, really funny.

When he uses the skill, a mysterious bBlack man playing the harmonica shows up in the background.

His theme BGM is really cool, but, unfortunately, it’s not included in the official soundtrack.

残念ですね。

The actual composer uploaded a video where he plays the song withon the guitar.

すごいですね。

TWho is the best Gaijin Character?

This makes sense, but it's probably best in my opinion to use a complete sentence.

He is an American maleguy who runs a store that specializes in Kabuto Borg machines.

"Male" sounds a bit too scientific, like it comes from a research paper. In normal speech, we don't refer to men and women as "males" or "females". It sometimes can be construed as rude or dehumanizing.

The main characters always hang around there, and he is a kind of a chaperonmentor to them.

Maybe I misunderstood, but I think the word "chaperone" may not really fit here. A "chaperon" is someone who drives people from place to place in a car. If you mean someone who guides them and gives them advice, I would say "mentor" works better.

He is an American malen who runs a store that specializes in Kabuto Borg machines.

Male is usually used as an adjective, so I think "man" is better in this sentence.

The main characters always hang around there, and he is a kind of a chaperone to them.

The bBest Gaijin Character?

Correcting capitalization.

He is an American male who runs a store that specializes in Kabuto Borg machines.

The main characters always hang around there, and he is a kind of aacts as their chaperon to them.

"Kind of a chaperon" may be a bit too casual in written contexts.

When he gets mad, he somehow he suddenly grows stubble, and his gaijin accent disappears.

This could go either way. "He suddenly grows stubble" or "he somehow grows stubble" are both correct depending on what you want to emphasize, but both at once sounds a bit strange.

Actually, hHe is also a Borg battler like the main characters.

"Actually" sounds like you're correcting someone, which isn't the case here.

When he uses theis skill, a mysterious black man playing the harmonica shows up in the background.

It’s hilarious, and nobody knows who the heck he is.

"Nobody knows who the heck he is" sounds kind of casual, but considering the tone of the essay, I could see it work. Just keep in mind it's not a formal essay phrase.

His theme BGM is really cool, but unfortunately, it’s not included in the official soundtrack.

BGM stands for "background music", so "theme BGM" is redundant.

The actual composer uploaded a video where he plays the song withon the guitar.