Going by the Replicant hypothesis he would have to be an earlier model than Roy as the fake memories seem to work better on him and he isn't as physically strong.

Near death Roy can leap with ease across the building, Deck only just manages to make to the girder.

If his function is to retire all the Nexus 6 models at large employing another replicant that nearly gets his eyes poked in by Leon seems a bit daft.

He only gets the job done because Rachel picks up his gun.

I think his real function is awaken Rachel to true emotional connection.

Which makes me not so concerned about her being the next step to perfecting the Replicant process.

She may not be superhuman like Roy but once she breaks through her conditioning she shows signs of genuine human behaviour.

There is a dichotomy inherent in the Tyrell Corp motto, more human than human is not human.

I wonder if she would pass the VK test after bonding with Deck?