What is the word or a phrase for people who are unable to be categorized into groups or stereotyped?
I am not looking for the word unique.
1.1 A person or thing differing from all other members of a particular group or set.
‘In groups it's often the non-expert, the outlier, or the person who isn't in charge who has the most interesting idea.’
You can describe someone as being atypical (i.e. not typical), which means unusual (another word you could use). You can also say that someone conforms to a stereotype or social norm; if they don't then you can say that they are nonconformist, or unorthodox.
If someone exhibits unusual behaviours, you can describe them as idiosyncratic.
If someone stands out, or is especially noticeable, because of their being different you can describe them as distinctive.
If someone is especially unusual then you can describe them as being one of a kind (One of a kind : a person or thing that is not like any other person or thing) - thanks to @EnglishStudent for pointing this one out.
among history's greats Leonardo da Vinci is often considered sui generis—a man of such stupendous genius that the world may never see his like again