This left me confused too. I believe there is a rule that you can omit the article "il, la, lo, etc..." when the noun being described is singular AND referring to family.
In that case, this phrase just got more intimate hahah! Hope this helps! Ciao!
I'm confused; I'm seeing a lot of people saying that because it's 'io' that this means the person saying it is female... I thought that 'io' means I, only. In my mind I had - this person said to her/him that she/he is hers/his. I went back to the basics of Duo's Italian course to see if I could find anything but I couldn't. Can anyone please highlight were I can find this explanation on when 'io' has a gender determination, thanks in advance.
Io as a word doesn't have one form for masculine and another form for feminine. It's not the io that tells us that the speaker is feminine It's the tua that tells us that a woman is speaking. After reading the whole sentence, we see that (to repeat what you thought) "this person (now we know she is female) said to her/him that she is his/hers." And her direct words to him/her were "I am yours."