I don't actually know Japanese at all, so I'm piecing the following together as best I can--if someone has a resource for this kind of translation, I would appreciate it as well as a specific answer, if someone has it.
If someone (female) has the name Kes in English, you would normally write it in katakana as ケス, "Kesu". However, this sounds the same as the verb 消す which means something like erase or turn off. So the transliteration of the name would sound either rather negative or rather scary (or both) as a name for a person.
Also, the transliterated name gives no suggestion as to the gender of the speaker, and it sounds like a verb which is awkward, so it would be better, possibly, to add a more characteristic feminine ending, the least bulky of which is -e (エ in katakana).
So the most conservative/simpleminded translation of the name (e.g. made by a child) might be ケスエ, pronounced Kesu-e? Does that work at all? I think one could write it straightforwardly in hiragana, but maybe not in kanji (or is it just 消すえ)? And am I correct about the connotation it would have? (Would an alternate ending avoid the connotation, or is it too obvious? In English, it doesn't really matter if you name someone Destroya or Destroyelle or Destroyina...it still sounds very destructive, even if it is clearly a name.)
If the translation were made by someone who is not a child, would there be a more idiomatic way to do it which would avoid the unfortunate connotation? Is there a standard practice for this situation? (Of course another option would be just to abandon the nickname and use something else.)