Blazblue: Phase Shift 4 Chapter 2

Trinity had no idea who Terumi really was, where he came from, or what about him Nine feared enough to put Mind Eater on him. All she knew was that they needed Terumi's knowledge in order to have a chance at beating the Black Beast and that, apparently, they wouldn't have been able to get that knowledge if they hadn't restricted his free will.

Nine wasn't the type of person to simply shackle someone and force them into her service for her own selfish benefit, so there must have been a good reason for her to take control of Terumi's actions. It's not like she didn't have some negative feelings about Nine not telling her the whole story, but she wasn't going to go against Nine's wishes just because of that.