A Sybil, named after the famous psychiatric case of Shirley Mason who reportedly had 16 personalities, and popularized by the book and movie "Sybil" is somebody who acts moody and irregularly. In a self-description, it may refer to a feeling of being "not quite oneself".
Just an hour ago, she seemed fine, now she's lost her temper again. She's a real Sybil.
Someone who tends to exaggerate their accomplishments to impress others. There may be a kernel of truth in their claims -- making it difficult to call 'bullshit' on them, not to mention the social risk involved in doing so -- but their exaggeration extends far beyond what might be considered remotely truthful.
More often than not, a dorky male with no real accomplishments to their name.
Jill: I've taken up learning the violin as a hobby.
Jack the embellisher: Oh, well I used to be a concert violinist.
Jack's friend: Lay it off dude, you learnt the violin for 8 weeks in primary school, and gave it up because you found it too hard.