Harriet Tubman (voiced by Millie Davis) was an American abolitionist and political activist. Born into slavery, Tubman escaped and subsequently made some 13 missions to rescue approximately 70 enslaved people, including family and friends, using the network of antislavery activists and safe houses known as the Underground Railroad.
She appears in the episode I Am Harriet Tubman, as well as an Ordinary People Change the World book.
Since she was met in 1833, she would have been 11 years old when the trio met her.
Although at that time Xavier says "it's her, Harriet Tubman", she would not have answered to that name during their first meeting. She was born Araminta Ross (her parents were Ben Ross and Harriet Green) and probably would've answered to her childhood nickname "Minty" at the time.
She only took on the surname Tubman in 1844 when she married John Tubman at age 22, so she would've answered to her better-known name during her 2nd meeting with the Riddles in 1857 when she was 35. The changing of her name from Araminta to Harriet also happened after the marriage, so the way closed captions refer to her as "Harriet" is inaccurate.
This historical inaccuracy is especially glaring when Brad addresses her as "Harriet" and she answers to that name even though it was over a decade before she would actually use it.
Xavier points out a statue of her at the start of the Secret Movie.