A bank.
But if Tether is a bank, then UST isn't a 'stablecoin' it's a deposit. And Tether is engaging in what banks have done for centuries, that is increasing the money supply through credit. They are doing it in an illegal and unregulated way which most likely will result in tears and recriminations. But ultimately there is nothing new under the sun.
Edit:
I was wrong in what I wrote above.
Tether is acting as a bank, and yes UST's are simply deposits, but all those people correcting me are right. Their assets (at least nominally) should match or exceed their libailities.