Frank Gianini (aka Mr. Gianini) is Mia's algebra and homeroom teacher. He dates her mother, Helen, and marries her when she becomes pregnant with Mia's half-brother, Rocky Thermopolis Gianini.

He plays the drums, which annoys the neighbors. He is unfazed by his stepdaughter's fame, and treats her the same as every other student during class. According to Mia, he also "has a thing about cell-phone usage during class time" (as Lana finds out the hard way in Princess in Waiting). Mia is very appreciative of his after-school algebra review sessions. Though their relationship begins awkwardly, Mia and Mr. G seem to get on well after the seventh installment. After it is revealed in The notebooks from a middle school princess royal wedding disaster that he dies and his wife Helen Thermopolis marries Mia's father Prince Phillipe