"Who are you, friend?"

Translation: Cò thusa, a charaid?

[RDM18]

Minor question - would people use the formal even if the person is not older, simply because you don't know them?

[jan_Awikiwa]

yes, you would use the formal to show someone respect, or anyone that you are not on close terms with. however, if you are a lot older than them, then you can and should thu, but if you're of a similar age, use sibh.
hope this helped

[Wolfie1026BF]

How would you say it then, if using "sibh"? (I am just beginning this chapter.)

[e.freed.2432]

I believe it would be "cò sibhse?"

[Latie20]

What is the difference between "thu" and "thusa"? It's the first time I see "thusa".