Thou correponds with I, he, she, we they and thee goes with me, him ,her, us, them.
As far as I know English has never had a separate formal Second person as such (unlike German and the Scandinavian languages), but you seems to have replaced thee thou and ye.
"I" is also by definition singular, but it's not "I is". "Thee is" is about as grammatical as "you is" or "he am". But then in many parts of England people say "I be, he be, she be, we be, you be and they be". Rules am meant to be broke.