"I will not allow anyone to harm you, Lydia."

And that is a promise, as sincere as it's possible for him to be. Daneel maybe has her halfway in a hug already, and it's far too easy for him to pull a little closer. He's not accustomed initiating hugs. It's not his place, and physical contact means far more to humans than it does to him. Now might be an appropriate time to try -- but it's a gentle effort, should it not be welcome.

"If he attempts to harass you, I will make him stop."