'Shiro, don't give away that you're a replicant.'
Sometimes, Shiro wasn't sure if it was more important to hide that he was a replicant or that he was very, very involved with Kokuyo. He didn't really understand it, himself - the latter, at least. Kokuyo had explained why it had to be kept quiet - and Hachi had confirmed that he and Saeki had to keep their own relationship secret or else there might be trouble at work.
They were still experimental, after all, and Shiro didn't want to risk being able to stay with Kokuyo.
Still, some days it was easier to keep secrets than others. Groceries and household shopping were frustrating when Shiro didn't have his own bankcard and had to rely on darting to ATMs with Kokuyo's. At least Kokuyo had taught him the basics of that.
And frequent shopper cards... Shiro had politely refused at first because the forms asked too many questions he couldn't answer. Eventually he just took one home to Kokuyo and asked how to fill it out.
For starters, Shiro didn't have a last name. Not until Kokuyo just told him to use his. And together they came up with other answers to hide their secrets, at least for the time being.