1. What does it mean?
본캐 shortens 본캐릭터. In games it is the character you mainly play; outside games, it can jokingly describe your usual job or central public identity, in contrast with a 부캐.
Also written or said as bonkae
2. The nuance
A character and an account are not necessarily the same thing. Calling one role 본캐 does not make hobbies or other roles fake; it identifies which one is being treated as primary.
3. Examples
“새 캐릭터도 재밌지만 장비는 본캐부터 맞출 거야.”
The new character is fun, but I'll gear up my main first.
“주말엔 공연하지만 본캐는 도서관 직원이야.”
I perform on weekends, but my main job is working at a library.
Sources
Sinjoeo Korean Slang Dictionary
References support the meaning and usage. Example sentences are written for NoCapWiki.