1. What does it mean?
부캐 is short for 부 캐릭터. It can mean a secondary game character, or an alternate persona used in entertainment, creative work, or a different part of everyday life.
Also written or said as 부 캐릭터, 부캐릭터, bukae
2. The nuance
The contrasting main character or persona is 본캐. Having a 부캐 can be playful or practical; it does not by itself imply deception or a clinical condition. A hobby can be described this way without becoming a second job.
3. Examples
“본캐는 궁수인데 부캐로는 마법사를 키우고 있어.”
My main character is an archer, but I'm leveling a mage as my alt.
“회사에서는 조용한데 주말에는 밴드 보컬 부캐로 살아.”
I'm quiet at work, but on weekends I have an alternate life as a band's singer.
Sources
Korea Policy Briefing: language and culture
References support the meaning and usage. Example sentences are written for NoCapWiki.