Fox posted a tribute to her "unusually handsome boy" as well, using descriptions like "magical and haunted" and "ethereal and dangerous" and writing, "the journey will likely be perilous but there is no destination without him."
Her post featured a slideshow of personal pictures showing off their connection.
For some fans, MGK's gesture brought to mind former couple Angelina Jolie and Billy Bob Thornton's alleged blood vial necklaces.
"You know those lockets you buy that are clear and you put a picture of your grannie in and wear it around your neck?" Thornton said in 2014, addressing the chatter years after their early 2000s relationship. "She bought two of those. We were apart a lot because she's off making Tomb Raider and I'm making Monster’s Ball. She thought it would be interesting and romantic if we took a little razorblade and sliced our fingers, smeared a little blood on these lockets and you wear it around your neck just like you wear your son or daughter’s baby hair in one. Same thing. From that, we were wearing quart jars of blood around our necks. And we were vampires and we lived in a dungeon."
While it's unclear whether Machine Gun Kelly and Megan Fox were inspired by them, you can see the couple's Valentine's Day photos here and here.