Word for “not having a definite form”?

Is there a word for something with a constantly changing or not easily identifiable form?

Example: If a person sometimes acts shy but other times is more extroverted, so you’re not sure how to describe their personality — is there a word for that state of not really knowing what category a person or thing falls into?

Solution 1:

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. [Collins Dictionary]

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.

Solution 2:

Some suggestions:

Ephemeral Lasting a very short time; short-lived; transitory.

Volatile Changeable; mercurial; flighty; fleeting; transient.

Elusive Eluding or failing to allow for or accommodate a clear perception or complete mental grasp; hard to express or define.