"Keeper" isn't really a direct translate, it's rather a term eclipse/ss came up with, it's just to make it easier to say.

Kaikin/Keeper it means when directly translated think it is "Forbids and Bound".

Now when this happens, I do remember there is something Rammie/Corpse Retriever try to tell Yuji "Not only you do not disappear, but you're..." , there was something Rammie was going to tell Yuji but did not.

Well I guess it was hinted.

That was probably "one" of the reason Bal Masque didn't take Reiji Maigo from him at the end of the first season.