While English has long left it’s thee, thou and ye forms behind in Shakespeare and prayer books, in favour of a universal ‘you‘, Italian like many other European languages still retains an informal and formal, singular and plural code when addressing someone. This means that there are three (some say four) ways of asking someone, ‘How are you?’ for example.
informal: “Come stai?” (abbreviated from “come stai tu?)
formal: “Come sta?” (from “come sta Lei?”)
plural: “Come state?” (abbrev. “Come state Voi?“)
Consequently personal pronouns are almost always dropped in the reply so the answer would usually be “Sto bene grazie” (I’m well, thankyou) or “Stiamo bene” (We’re well). They may however be included for emphasis or contrast so a possible answer might be: “Io sto bene, e tu?“. (I’m fine, how about you?)
A fourth form of address “Loro“, a formal “third person plural” pronoun, is still sometimes used although fairly uncommon in everyday language.