Can you give any more information? The Dresdenverse is pretty open in terms of what you can play. The most likely way something will feel "non-Dresden" would be the motivation of the character or how it fits in the context of the group as a whole.
If the character is motivated by selfish desires and is cruel to others, they're not going to fit well in the Dresden Files world, where PCs are responsible for making their own motivation to put themselves at risk for the sake of others.
Similarly, if you're playing a group with a werewolf, a wizard, and a White Court Vampire, it's going to be very hard to incorporate a zealous monster hunter who believes all monsters and magic-users are evil and should be destroyed.
If your situation is like either of these, then you should be able to fairly easily explain to the player that their character might have trouble working with the other PCs, and that will cause delays in play. See if they're willing to tweak their concept a bit, rather than saying no outright.