"Theta Sigma. My name back at the Academy." The Doctor figured he might as well state it right now and out loud, he'd seen the look Jack gave him. By now he almost thought of Theta as a separate person, but the way Koschei... The Master, but Koschei now, was talking to him and calling him a name he hadn't heard in too long... It made him feel just a bit like Theta again

"You shouldn't bother Jack too much, Kosch." There he went, the shortening, it already came natural. He had to get a grip on himself. "Come here, Jack. I'll fix up your vortex manipulator a little, if you're going off on your own I want you to be able to send out a distress signal when an amazon is crushing you between her thighs or something."