Kaat grew up in the Red Room with Yelena and Natasha. She’s always had a little crush on Yelena. Expect Kaat wasn’t ‘normal’. She had blood that was… black. She was born with it. But no one could know that. Dreykov made sure no one would know. He had a special serum to make her blood seem red (this process wasn’t painless). Dreykov ran tests on her frequently, it left scars on her back and the right side of her stomach. None of the other widows ever had any idea.