This has been discussed on comp.soft-sys.math.mathematica. The gist is that there are lots of Unicode characters you could use, e.g.
\[UnderBracket] (you could consult https://reference.wolfram.com/language/tutorial/LettersAndLetterLikeForms.html for a long list), but I'd strongly urge you not to do that.
Once you copy the code out of Mathematica, these will be expanded to the above long forms which will make your code very unreadable. I think there are ways to "copy as Unicode", but a lot of these are in Unicode's private use pages, which means they will almost certainly not render properly anywhere outside of Mathematica.
camelCase is such a widespread convention in Mathematica, that it's probably not worth the trouble working around that just to match your programming style in another language. Mathematica's syntax works differently from C's, and I don't think there's a lot of benefit in trying to make Mathematica look like C. You'll probably save yourself a lot of trouble by simply embracing Mathematica's own conventions.