The Butler might be surprised what sort of serial numbers Haruto might be able to recall. He notes that the Butler said that's what their family 'called' them, past tense. He won't pry. Just as he's appreciative that the Butler hasn't asked him any further questions about Project Integration.

"I suppose, so long as you're the only household robot here, that won't be too confusing." He pauses for a moment, knowing this would be the part of the conversation where he would give his name in turn, and trying to decide which name to give. He's been telling others that his name is Hideki Matsukawa, trying to catch out anyone who might recognize the name as the donor body his consciousness had been housed in. But... He's already confirmed that the Butler doesn't know, and isn't a part of Project Integration. He inherently trusts the Butler more than he has trusted anyone else he's met here so far.

And so... "You can call me Haruto. If you ever need me."