I am writing a detective story and one of my characters is a Russian soldier who deserts the colors in WWII. I want to give him the name Alexei Kamenski but I am not certain about my spelling it properly. Should it be ‘Alexey’ or ‘Alexei’? “Kamenski’ or ‘Kamensky’ or whatsoever? Thnx in advance;-)

correct variant is ‘Aleksey Kamenskiy’
by the rules you should translitterate EVERY letter

Batty’s variant is correct but in fact there is more than one way to transliterate such cases.
Alexei Kamensky (Kamenskii, Kamenski) is also possible.