Temperamental is what you're looking for.
A temperamental person is someone whose behaviour is unpredictable or whose mood changes suddenly.
Capricious and mercurial can also be used for such kind of person.
Mercurial: If you describe someone as mercurial, you mean that they frequently change their mind or mood without warning.
Capricious: having a tendency to sudden unpredictable changes of attitude or behaviour. [Collins Dictionary]
Due to their capricious and erratic behaviour, they are unclassifiable.
If a person sometimes acts shy but other times is more extroverted
Ambivert: a person whose personality has features typical of both introverts and extroverts.