Taking it literally
Flirting on messaging apps may be entertaining, but it also raises questions about the other person’s authenticity. That’s why some people request that their conversational partner send them photographs throughout the conversation.
The other person, however, doesn’t have to send a picture of themselves if they don’t feel comfortable doing it. So when this person asked for a “pic of u,” the crush decided to be clever instead and sent an actual picture of the letter “u” on their keyboard.