Is it okay to use a character name from a tv series to your novel and publish it online?

If it is only a first name OR a last name and either of them is common, then yes, you can use the one name. If you use the entire name or a name that is instantly recognizable as a character from a TV series then you are plagiarizing and could be sued for theft of intellectual property.
TV writers and production companies copyright their scripts which includes their characters and character names. So if you name a character Dr. House or Ben Cartwright or Uncle Buck, you are breaking the law.

No.

If it's Buffy, no. If it's Cathy, that's ok.