I cannot place Kes' accent. What is it? Is it based on any non-fiction accent? Is it the accent of Jennifer Lien from her daily life? Or was it a choice to make this the accent of the character?
Jennifer Lien played the character of Valerie Fox in The Critic. Her accent seems very similar to the one seen in Star Trek: Voyager (albeit slightly more...sultry).
Lien is apparently a native of Chicago, Illinois and her accent reflects this.
Jennifer Lien has used a similar accent in at least one other show (The Critic). Lien is a native of Chicago, Illinois.
However, it is hard to tell from her Voyager accent that she is from Chicago. Her Voyager accent does not display the Northern Cities Vowel Shift and she may be attempting to sound like she is not from Chicago.
Lien played Hannah Moore in Another World in 1991. There, the accent she used incorporated aspects of a typical Chicago accent (see especially around the 9:45 mark of this episode: "I got outta there", the vowel in got is shifted towards an a like in cat).
I have been unable to find any source that states what non-fiction accent she has/uses in Voyager (and in The Critic).