Charlotte is an older woman who Stephen met on his first flight, from Charlotte to Detroit in 2008, on his way to see Mallory for the first time. She is presumably 77 to 78 years old (as of June 2018) and lives in New Jersey.
She has never been seen on the vlog before, but Stephen has mentioned her at several instances.
In June 2008, Stephen happened to sit next to Charlotte while on his way to Wisconsin to see Mallory for the first time on a flight from Charlotte to Detroit. Stephen started to talk to Charlotte about Mallory and his relationship with her. By the end of the flight, Charlotte wanted to know the outcome of his trip to see her, so she gave him her address and they split ways. A week later after seeing Mallory, he wrote her a handwritten letter, and sent a picture of them together. Stephen would not hear back from Charlotte for three years.
Three years later in June 2011 (Day 573), Stephen finally received a letter from Charlotte. In the letter, she explains that her husband had died, her children refuse to speak to her and her brother's grandchildren are taking her to court. However, she thinks that Stephen and Mallory make a great couple. She ended up predicting Stephen and Mallory's marriage; at the time they were less than two months away from their wedding. He sent another letter to her shortly after the wedding was over.
Half a year later in December 2011 (Day 765), Charlotte just so happened to be stopping by Myrtle Beach so they decided over the phone to have lunch together. According to Stephen, she was doing fairly well.
A year later, Stephen sent her another letter wishing her Merry Christmas during Christmas 2012, but never heard back.
Three years later in January 2016 (Day 2249), Charlotte called Stephen's dad, and left a voicemail with her phone number. He then called Stephen, saying that he thought it was the same Charlotte that Stephen had spoken to while on the flight to Detroit, as well as during her stay in Myrtle Beach.
Once Stephen dialed the number, he quickly realized that it was the same Charlotte. They ended talking to each other for two hours. She also got a chance to speak to Mallory as well.
Though they have not talked to each other since then, Charlotte told Stephen that "[she'll] call [him] when [he] least expect[s] it."