“Everyone at the top of Gilead has lied to us: God isn’t what they say. You can believe in Gilead or you can believe in God, but not both.”— Becka (as Aunt Immortelle) reflecting on her Gileadean education 
Becka Grove is a character in The Testaments. She is a Daughter (though the child of a dentist rather than a Commander) who was a classmate to Agnes when she was growing up. She is later made an Aunt and referred to by her rank as Aunt Immortelle.
Becka may be a shortened version of Rebecca, the wife of Isaac and mother of Jacob and Esau in the book of Genesis, the first book in the Bible.
Her name is more common than those of her higher-ranking classmates. This may indicate the cultural differences between Daughters of Commanders and their Wives, who typically have obscure religious names, and the daughters from Econofamilies, whose names are more commonplace.
As an Aunt, Becka chooses the name "Immortelle" from the allowed list. As names for Aunts are, but not usually, chosen to be comforting by referencing things women liked from the "time before," it likely references Immortelle, an overnight serum/youth oil by L'Occitane.
Agnes describes Becka as having "barely-there eyebrows."
She is then later described in her later years as being a thin, pretty girl with large eyes by Aunt Lydia.
Becka was taken away from her parents and placed with the high-ranking Econofamily of Dr. Grove and his wife. As a child, she was severely sexually assaulted, which led to her aversion to marriage later in life.
Despite not being the child of a Commander and Wife, Becka was educated as a Daughter due to her father being a skilled professional - a dentist. She seldom asked questions at school, but once asked Aunt Vidala what "adulteress" meant. Vidala did not answer her directly, but threatened that if Becka found out, it would meant her being stoned to death or ending up on the wall.
Becka cried in class when Aunt Vidala taught them the story of the Concubine of a Levite in Religion.
Her father was extraordinarily allocated a Handmaid due to his highly skilled profession. Becka wanted a sister, but was horrified when the Handmaid gave birth to an Unbaby without a brain.
When she reached menarche, she went to Rubies Premarital Preparatory. Her family had arranged for her to marry the teenage son of a Commander, but Becka grew greatly worried at the prospect of her wedding, and slashed her arm in class. Her suicide attempt failed, and the Aunts could not talk her into agreeing to a marriage. She then became a Supplicant, training to be an Aunt and taking the name Immortelle.