The meaning of ki in this sentence

"キめす"

and

"キめさせよ"

Why is ki in katakana by itself? Is there a specific reason for that? What does this sentence say?

Are they Japanese words written by a Japanese speaker?

We need more context. That said, kanji is often replaced by katakana especially when the word is meant to be slang-ish or colloquial.

Not a “usual” Japanese word/phrase at all.
Any context, like some comics characters or something? Can you give the whole text/dialogue?

The person that asked for it only gave me these 2 crops:

https://imgur.com/a/sOLndIL

I'm totally lost lol.

I’m totally lost as well.

At least, the first one should be キめる、not キめす。

“kimeru” originally in colloquial language (usually written as キメる though) means to get high (on whatever substance), but has other meanings as well, such as completing a winning move in martial arts/combat or make an excellent pose, etc. I have never seen it written with just the “ki” in katakana.

You’d need more context.

You can't translate anything accurately without context. That said, my guess is that this is a rape scene of some sort, and the meaning of the word is the "high on something" one. I agree with AK that, when using the term to mean "high on", it's more common to write both キ and メ in katakana. But since you can also say きまる, I wouldn't be surprised if the writer decided to use katakana only on the キ.

As others have said, it's necessary more context to give an appropriate translation.

That said, the word きめすぎ means someone that is overdressed. It refers to someone who tries to look cool but ends up with the exact opposite effect.

See this image for an explanation:

https://cdn.hinative.com/attached_images/77688/a168fb778ad420c9b173555...