The expressions kind of or sort of mean “rather,” “partially,” or “somewhat” are nonstandard.
Both expressions literally mean “type of” or “variety of.”
Incorrect: The child felt kind of lonely.
Correct: The child felt somewhat (or rather) lonely.
Correct: The kestrel is a kind of falcon.
(A type or variety of falcon)
When using kind of or sort of always use a singular object of the preposition of. If the object is plural, then use kinds of or sorts of.
Examples: What kind of salad is this?
(Kind and salad are both singular.)
There are all sorts of fruits in this salad.
(Sorts and fruits are both plural.)
If you are using an expression like kind of, sort of, type of, or variety of, then you are putting the person or thing you describe into a category. Therefore, such expressions should not apply to one specific individual of the type.
Incorrect: He is the kind of leader we need.
(You are referring to a specific person in the subject and to a category in the predicate.)
Correct: He is like the kind of leader we need.
(You are referring to a category, and he is an example of someone in the category.)
Correct: He is the leader we need.
(You are referring to a specific person, not to a category.)