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.
In addition to Luffy and Garp, other characters could also play important roles. For example, some of Luffy's crewmates like Zoro or Nami might be present when Luffy calls Garp. They could add their own perspectives and reactions. Also, there could be some marines around Garp when he receives the call, and their reactions to Garp's decision to go to Luffy could also be part of the story.
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.