The only time I've come across this name was reading Mary Webb's 1924 novel, Precious Bane. Kester Woodseaves is the love interest of the main character, and seems quite a positive association; he's hardworking, strong, brave and funny, like the main character herself. The story is tragic, but also incredibly beautiful.Personally the name sounds completely masculine to me. Strong but friendly, too. Despite the similarity in the sound, it didn't make me think of Lester as another commenter mentioned. Esther vaguely came to mind when I first read the name, but the harder K at the beginning made it sound much more masculine to me. Most of all it made me think of 'kestrel', which is an association I really like. I think it's an underused, yet very usable name. :)
Kester sounds like a more feminine name to me. It is similar to the name Esther and it could be shortened to Kess which is a cute nickname for a girl. It just doesn't come off very masculine to me.