Can String be a subtype of Character in a programming language?
Character and String are conceptually very, very different. Google for "inheritance vs. composition" - you seem to think that Character and String should be connected via inheritance, but in reality a String is composed of 0, 1 or more Characters. 

And to throw a spanner in the works: Strings are often large, and String operations must be very efficient. Therefore, even though conceptually a String is an array of Characters, it will not be implemented as a sequence of objects, but as an array of somehow simpler elements, for example UTF-16 codes. 

And then you can think about how to implement Character efficiently if you use Unicode and an emoji character can easily be made up of a dozen Unicode code points - but the huge majority of Characters doesn't need that complexity, and you want an implementation that is space efficient and fast.