Miss Temple, the teacher in charge of Lowood Institute, is the only person able to protect the girls at the school from the cruelty of Mr. Brocklehurst. Miss Temple gains Jane’s loyalty forever when she takes steps to find out whether Jane really is a liar, as Mr. Brocklehurst and Mrs. Reed claim, or not.
There’s really not much to Miss Temple’s character, but she’s important to Jane’s development as someone who takes the trouble to care about the truth and someone who encourages Jane to apply herself to her education. She also has a semi-allegorical aspect; for eight years, she basically is Jane’s place of refuge (get it—temple?), and, when she leaves Lowood, Jane is ready to leave, too.