Who is Thaddeus Isben from After Ever After and what is their importance?

Thaddeus Isben is Jeffery's best friend. He has had more serious physical side-effects of the cancer treatments he endured as a child and his limp is so pronounced that he gives up and depends on a wheel chair. Tad is self-confident to the point that he seems pushy, but it's mostly an act to cover his insecurities. Though Tad tries hard to come across as a selfish person, he asks to tutor Jeffery when it becomes clear that Jeffery is having trouble in math and may not pass the eighth grade because of it. Tad then makes a point to walk out on a standardized test in protest, an action that brings a lot of attention to the testing practices and eventually eliminates the use of testing as the sole means of determining if a student passes a grade. Tad is nearing the end of his eighth grade year when it's discovered that he has developed a new form of cancer. He doesn't tell Jeffery about it for a long time and seems to accept that he probably won't live through the next attempt to save his life. Tad dies on a day when Jeffery is riding a bike-a-thon with the money being donated to research in Tad's name. Tad continues to be an influence in Jeffery's life even after his death and Jeffery can imagine Tad's words on many subjects.