"Who are you?"
a is a relative particle, introducing a relative clause, here there is no relative clause to introduce with it.
Look for example at another question:
dè an t-ainm a th’ ort what is your name?, meaning literally: what-is the name that is on-you?, because in Gaelic people don’t speak about their names but rather names that are on them.
This question has two parts:
- dè an t-ainm…? – what-is the name…?
- a th’ ort – that is on-you
and a th’ ort that is on you is a relative clause, with its own verb (tha, is), a second sentence of its own embedded in the question.
If you just wanted to ask what is the name?, you’d just ask dè an t-ainm? without any a nor any relative clauses.
The same happens here, you just ask who-are you?, cò thusa?. No relative clauses.
You would need one if you wanted to ask something like who is the one that is there?, that would be something like: cò am fear a th’ an-sin? with a th’ an-sin meaning that is there.
(But then, this a might not be written after vowels, co eg. dè tha… instead of dè a tha…, or cò tha… instead of cò a tha…, etc. – so you might find sentences where you’d expect the relative a grammatically, but it wouldn’t be explicitly written out, just as vocative a isn’t written before vowels: Anna or Eòghainn instead of a Anna, a Eòghainn)