Bob Banner is Bella Swan and Edward Cullen's Biology II teacher during their junior year in the novels. It is the class where they first communicated. In the Twilight movie, his counterpart is Mr. Molina, and his Life and Death counterpart is Mrs. Banner.
When Bella Swan arrives at his class on her first day at school, he hands her the textbooks and immediately shows her her seat rather than making her introduce herself to the class. Bella senses that they were going to get along because of that.
One week later, he assigns his students to partner up and separate the onion root tip cells into the phases of mitosis and label them. To his surprise, Bella had already studied a similar subject back in Phoenix. He had read through textbooks all week on the subject.
Behind the scenes
Stephenie Meyer said that in Twilight, he had a slight crush on Bella.