If Katniss didn't trust Derek, she wouldn't continue to sneak into his bed every night. She wouldn't let him hold her at her most vulnerable if she didn't trust him. He's become important to her. Becoming a little more important to her every day. If she should introduce anyone here to Prim, it's Derek. Moreover, keeping secrets from her sister has never been easy.

At his question, she briefly wonders if she had done the right thing telling Prim about Derek. Then she dismisses it. He had kissed her goodbye at work the previous day. That makes it more public. They had kissed at the gala, too, hadn't they?

"Soon," she answers confidently, then stealing another kiss because he said she could, didn't she? Whenever she'd like. Given how tempting it is to give into her self-hatred and self-pity for the rest of the night, distracting herself with Derek's mouth seems like a much better solution. "I messed up tonight," she continues, quieter and more somber. "And I could've lost her - or you - because of it." Another kiss. "I don't want to keep things from her."

Besides, she wants Prim to have someone to look after her if something happened to Katniss. Katniss trusts Derek to do that job.