She's definitely a loving girlfriend, willing to do this just to make him happy.
Also, her grandfather is such a wingman. Just a simple suggestion/wish of his and bam, they become a couple.
Great ending though! Yeah...I suppose a collar would indeed be a sign of a...stable and healthy relationship. Still, reporting to good ol' wingman grandpa, eh? I suppose his next move will be to ask to see his granddaughter's wedding, and then to see his great grandchildren...
Seems (if I remember the term right) "Ojii-san" decided right there to ensure he sees his great-grandchild (or more) while he's still in this world. That's some serious responsibility to place upon a healthy young man, together with the female protagonist (Miyuki) awakening to a (sort-of) "S&M" fetish.
For now, Do Your Best, Kengo-san (you Lucky Bastard)!