I don't have enough context to determine exactly who 'garp' is. It could be a newly created character by the fanfic writer, or it might be a character from a relatively unknown story that is being incorporated into the 'ace fanfic'.
There's Roberta Muldoon. She is a transgender character, which was quite revolutionary for the time the book was written. Her friendship with Garp adds depth to the story. Also, Helen Holm, Garp's wife, is an important character as she plays a significant role in Garp's personal and family life.
The main themes include gender roles. The female characters in the book often defy traditional gender norms. There's also the theme of fate and how characters' lives are shaped by events beyond their control. Violence is another theme, as it impacts the characters' lives in different ways.
One of the main themes is the search for identity. Garp is constantly trying to figure out who he is in the complex world around him. Another theme is the role of family. His family relationships, including his relationship with his mother, wife, and children, are full of love, conflict, and growth. Also, the idea of gender roles is explored, as Garp interacts with strong female characters and navigates through a society with certain expectations of men and women.
Garp himself is very memorable. His journey as a writer and a family man is full of interesting twists and turns. His mother, Jenny Fields, is also quite remarkable. She has her own strong beliefs and a very independent personality. And Helen Holm, Garp's wife, with her own dreams and the challenges she faces in her relationship with Garp.