An Unexpected Guest

The title feels a little empty without 'an' or 'the'.

When I started playing Priest Simulator, someone whose name that was so unique followed my channel.

Also:
* When I started playing Priest Simulator, someone whose name that was [very] unique followed my channel.

Their name was “YankeeGyaruJKWakaraseNTR”, and it was hilarious.

Don’t ask me why.

Yes, I did look it up on Google and I was not disappointed.

I tried to say thanks you to this new follower, but I couldn't help laughing.

If you're not quoting the "thanks" part you can do this to make it more clear:
* I tried to thank the person for following, but I couldn't help laughing.

I asked them,... I mean,... her, “Are you Oneechan?”

The ellipses (...) are a better way to convey shuddering in my view.

“big boin boin nekomimi oneechan desu wa”,

Sshe answered.

Because this is 'technically' a journal entry, bring the quote into the sentence.

This name is totally her style, but I didn't notice that.

While this isn't incorrect per say, I think you can improve it.

* A name like that is totally her style, but I didn't quite make the connection.

She whas only been witnessed appearing in live streams on YouTube and there was no info that she appearedhad shown up on Twitch before.

On this one, 'witnessed' is usually followed by another verb or a description of what occurred:
* He [witnessed] the highest pogo stick jump for the record books.

She was so funny as always, andthat my brain function wasgot debuffed for a while.

If you'd like to say that 'she's always so funny', add it to the previous sentence.

An Unexpected Guest

When I started playing Priest Simulator, someone whose name was soreally unique followed my channel.

'so' sounds a bit off here. 'really', 'very', 'quite' are all good replacements.

Their name was “YankeeGyaruJKWakaraseNTR”, and it was hilarious.

I tried to say thanks to this new follower, but I couldn't help laughing.

Alternate phrase, "I couldn't stop myself from laughing".

At the same time, my brainI started noticing something, but couldn't figure out what it was exactly.

This name is totally her style, but I dihadn't noticed that.

She wasI had only witnessed her in live streams on YouTube and there washad no infodea that she had appeared on Twitch before.

She was so funny as always, and my brain function was debuffed for a while.

I had to look up debuffed. I guess it's only used in video games but it looks correct!

An Unexpected Guest

We usually use a particle to introduce a noun in a title.

The name was “YankeeGyaruJKWakaraseNTR”,” and it was hilarious.

“big boin boin nekomimi oneechan desu wa”,” she answered.

She answered.

I deleted this and appended it to the previous sentence.

She whas only witnessedbeen seen in live streams on YouTube and there was no info that she has appeared on Twitch before.

She was so funny as always, and so my brain function was debuffed for a while.

ハハ！今文、好きです。

The name was “YankeeGyaruJKWakaraseNTR”," and it was hilarious.

i genuinely couldn't find anything else to correct so this is it chief :'v

i was taught that punctuation had to be inside of quotations like i corrected. i think this might just be an american english thing though, i've seen other people in europe say they were taught to write it the way you had spelled it

She was so funny as always, and my brain function was debuffed for a while.

this is correct but sounds unnatural in a way that i'm not sure how to rewrite it without changing the meaning. i think "and my brain was mush for a while" is the closest in meaning but it still doesn't entirely sound like something you'd say there... perhaps "and i couldn't think straight for a while" ?? idk