Some translations for Kerl are fellow, bloke, guy, chap and dude. Most of them are (somewhat) gender-neutral. What about Kerl?
It is mostly used for male persons, but may be used for women as well. Be careful though, because in addition to this rather neutral meaning, it also has the meaning of man, which you can't say to a woman obviously, unless you want to insult her or want to explicitly express that she's (in some regards) like a man or a good Kumpel, like jemand mit dem man Pferde stehlen kann.
Another meaning of Kerl is Liebhaber (lover).
Note that the definition from Duden is important: Kerl is used for females in the sense of "(durch bestimmte [positive Charakter]eigenschaften charakterisierter) Mensch" - in the sense of Mann, Liebhaber or Prachtexemplar, it is not, at least not without causing offense. From the Grimm:
For these reasons, a woman may not be a Kerl, but she may be a netter Kerl. However, the positive adjective is sometimes omitted, even though the gender-neutral meaning is intended. Of course, it may be easier to avoid offense by just not using the word. But that would be too easy, wouldn't it? ;)
As OregonGhost mentioned in a comment, the key to this is in the adjectives. Without adjectives, "Kerl" can only be applied to males and basically means "guy", and can but need not have negative connotations, as in "Was ist das denn für ein Kerl?". The negative use is relatively common, whereas the neutral use synonymous with "man" is obsolescent except in certain dialects. With positive adjectives, however, as in the Duden entry OregonGhost cites, "Kerl" can also be applied to females, though this is not very common.