Theresa Wayman is a guitarist and vocalist of the indie rock band Warpaint. Under the name TT, Theresa releases her solo records.
Her nickname TT has a rather sweet backstory:

I have a kid and his dad calls me TT. When my son was younger he had a friend whose name was also Theresa, and they called her TT. She had these thick glasses and I thought she was adorable. I figured that if there was any way I could be called TT then I’d make it happen.