Every human was born with the first words their soulmate would say to them stained on their left arm.
Clara Oswald's was "What are you?"
What a strange message.
Gallifreyans weren't born with words. Or any kind of soulmate-indicative marking of any kind.
Time Lords weren't born to have "soulmates", as it were. Growing up amidst the human, the Red Lady was very confused by her unblemished skin. Wasn't she supposed to have someone too? The others around her were finding their soulmates left and right. What was wrong with her?
Decades later, she would find out the truth about her origins. But in her youth it was very saddening to think she would always be alone.
When she took on her renegade life, traveling time and space with her friends the Doctor and the Master, and then later on when she branched out by herself, Red assumed that the reason she didn't have words was because she simply moved too fast to ever find a soulmate. She didn't plan on getting stuck.
Or meeting Darla.
When Red first uttered the words "Okay, okay. I don't have any weapons," the warrior's dreamy brown eyes lit up in sudden revelation. She gasped, lowering her gun. She stripped off her jacket, rolled up the sleeve of her turtleneck, and showed Red the words branded into her arm. Red had no marks, so she obviously couldn't confirm their connection. But as time went on, they fell deeply in love, and Red felt things in her heart that simply couldn't be anything else-this was her intended.
Red's soul was ripped apart when Darla died.
The Red Lady tried to tell herself it had been a fluke. Darla couldn't be her soulmate anyway. Red didn't have one. It eased the pain…or at least, Red could fool herself into believing it did.
So when she stood in front of the spitting image of her lost beloved, Red couldn't stop the words tumbling from her mouth. "What are you?" she spat angrily, feeling that her pain was being mocked.
And Clara's eyes lit up in a very familiar way.