This article contains plot details from the latest book.
When Sophie was assigned a star-mapping project, and she bottled the light of an unmapped star, Elementine. Sir Astin was shocked. He tells Sophie that it is made of Quintessence, the fifth element and that she should hold still while he thinks of what to do. Eventually, Foxfire is evacuated, and Sophie must attend a tribunal because she has broken a serious law - one she did not know existed. At the tribunal, Sir Astin testifies that her list of stars was assigned randomly and the pattern was metallic stars. He said that he had no idea how she came up with an unmapped star and that he knows nothing about Elementine except for its name. Sir Astin gives Sophie a hundred percent at her final tribunal in book one because she has all of the stars memorized.
Mr. Forkle reveals that Sir Astin is one of his identities, in order to gain Sophie's trust. As Sophie's teacher, he gave her the star list that led her to bottle the light of Elementine, even though he claimed that it was not one of the lists he created at Sophie's tribunal. When Sophie asked why Sir Astin appeared so surprised when she came in with the bottle of Elementine, he says that he didn't think she would bring it to school. He forgot that Sophie wouldn't realize the light was dangerous because of her human upbringing. Mr. Forkle (as Sir Astin) wanted to bring out Sophie's hidden memories.
We find out that Sir Astin (or Mr. Forkle) was made up of two people. Sir Astin is technically still alive, though he is still "on a stargazing excursion"