The two spent the day together Saturday with Harper giving Kaylan a tour of Nationals Park and the team's facilities. She wore a custom No.34 Nationals jersey with her name on the back as the two played catch together and took swings in the batting cage.
In collaboration with the Make-A-Wish Foundation, Harper and Kaylan's time together will be featured on ESPN as part of the 10th anniversary of SportsCenter's My Wish Series, beginning July 17.
"Best day ever," Kaylan said. "It's hard to recreate."
Kaylan has been battling a kidney condition since birth and in 2015 she learned that she needed to have a transplant as soon as possible. Her mother, Summer, ended up being the person who gave it to her. Kaylan does have to take 24 pills every day to prevent organ rejection and she undergoes regular scans to monitor her kidney function. The transplant is not a permanent fix and she almost certainly will have future medical complications and could need additional kidney transplants.
But Kaylan is able to enjoy her life for now and spends it playing varsity high school softball Gaithersburg, Md. She also gets to Nationals games whenever she has the chance, including Max Scherzer's no-hitter on June 20, 2015. Kaylan named Harper as her favorite player, and the best part of the day was just getting the chance to hang out with him and talk baseball.
"Kaylan, she's a sweet spirit," Harper said. "I've never done anything like that. Doing something like that, it's a lot of fun. I'm very happy that I was able to do that and the whole team, the whole organization, they really took part in that. I can't thank them enough as well for letting her come in and do what she wanted to do."
Before Kaylan and her family watched Saturday's game against the Reds -- a 9-4, 10-inning loss to the Reds -- she got to throw out the ceremonial first pitch to Harper, who implored her to throw from the mound.
"It's been more than I could imagine," Kaylan said about her day. "It's been great."
Jamal Collier covers the Nationals for MLB.com. Follow him on Twitter at @jamalcollier. This story was not subject to the approval of Major League Baseball or its clubs.