Mackenzie is the 8-year-old girl that lives in the house Chandler and Monica buy. She doesn't want to move and is upset that her parents are selling the house. She has a bond with Joey because he doesn't want Chandler and Monica to move, either. The two briefly talk and she teaches Joey that if he really wants Monica and Chandler to be happy that he has to let them go.
Mackenzie appears to be slightly sarcastic and points out the flaws of Joey's quickly thought-up plan of telling Monica and Chandler that the house is haunted. She also seems smarter than most kids her age and when Joey points this out, she explains that it's because she reads a lot. This causes Joey to angrily say "Just when I thought we could be friends!" and leave the room, much to her bemusement.
When Joey tells Monica and Chandler about Mackenzie, Chandler says that there was an eight-year-old girl living in the house who died 30 years ago. When Joey gets scared of this, due to his fear of little girl ghosts, Chandler takes his comment back saying he was kidding. Joey later calls Mackenzie for career advice.
Mackenzie also has a pet stuffed bear named Bailey.