Stephen Beadman, 28, of George Avenue, Ibstock, pleaded guilty when he appeared via videolink at Leicester Crown Court.
The 15-year-old, from Leicestershire, was found dead last November.
Luke Harlow, also of George Avenue, admitted sexual grooming and two counts of sexual activity with the teenager.
Both men denied a charge of false imprisonment and will stand trial at Nottingham Crown Court on 8 June.
Police from across the East Midlands helped with the hunt for the teenager, after her disappearance on 13 November.
Kayleigh's disappearance prompted an outpouring of support for the family.
Vigils were held in the village of Measham, where she lived, and Ibstock where she had been a pupil.
Her body was found in a field outside Ibstock five days after she was last seen outside Ibstock Community College.
An inquest has heard Kayleigh died from head and facial injuries.
Kayleigh's family said at the time they were "heartbroken" and "grief-stricken".
"There are no words to express the way we are feeling," they said.
"We cannot even begin to thank the local communities and wider public for their messages of support which have helped us through the last week."
At Kayleigh's funeral in January, hundreds of mourners lined the streets of Measham as her coffin was carried on a white horse-drawn carriage.
Her favourite colour - purple - featured, with ribbons, flowers, and bows lining the route to the church.
Tributes were paid at the service by the headteacher of Ashby School, her brothers and sisters, and her parents.
Her family has previously said they plan to campaign to change laws surrounding the use of social media.
A man has admitted raping and murdering schoolgirl Kayleigh Haywood, who was found dead in undergrowth five days after she went missing.