Gokuama -Yonpakugan no Maid-san-

The protagonist, who was unconscious for over 2 years following a traffic accident, finally opened his eyes to see a maid watching over him by his bed in the hospital.

What set this girl apart from most people was her looks: she had huge breasts and a sharp look in her eyes that could exert terrifying pressure, but the real her was actually timid and had low self-esteem; and for some reason, she was extra submissive to him. He lost his memories from the shock of the accident, and while he didn't yet have a good grasp of the current situation, he took up her advice and they began a weird relationship living together under the same roof.

Even though there is mutual love between them, the maid insists that she and her master shouldn't get married with the thought,
"I must make master understand what a proper master-servant relationship is..."

And so she begins to apply effort in taking things in the wrong direction (from an equal relationship), like wearing a collar on a leash, or putting on restraints; the two of them will continue to get along, and this kinky play is something they'll even get hooked on.

(Source: DLsite)