[@Prince of Seraphs]

Shiro is a fringed case. Let's not mince words; Gren is obviously manipulating Shiro. On one hand, Gren doesn't expect Shiro to actively go against him. But at the same time, Gren doesn't exactly control Shiro's every move either. It's entirely possibly that at some point someone could convince Shiro to turn against Gren for whatever reason, most easily simply revealing Gren's past and the full extent of what he did for the White Fang (Pillage, Rape, Burn), in which case Gren has very little means to stop them except hoping that his friendship with Shiro is stronger than Gren's sins. Thus, Gren keeps an eye out not only for Shiro, but people Shiro hands out with. Using him to make friends helps cull Gren's possible enemies or root them out.

Now does Gren trust Shiro? No. But does he believe Shiro is useful and there could be a stronger bond between them? Sure, but it's not going to be easy. Gren has already taken the first step by revealing he's a cyborg, but now he's waiting to see what Shiro has to offer before Gren gives the rest of his secrets. It's selfish, but Gren isn't going to just give and expect nothing. He doesn't trust Shiro enough.