The Tranny is a mysterious person whom you meet in the Public Restrooms during the Betty's Valuable Item quest. She appears very feminine and when you find that she is locked in a bathroom stall and needs you to let her out, you can either take advantage of her or help her out.
If you take advantage of her, she'll start blowing you through the glory hole before inviting you in to have sex. When she takes her clothes off, you realize she has a dick. You can either fuck her in the ass or stop things there. After you're done, she runs off saying that she'll see you again sometime without giving you her name.
If you help her out and open the door, she hurries off and promises she'll repay you somehow in the future.