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'.
I'm not sure specifically what the 'Tekking 101 Manga Girl Story' is about as there isn't much information given just from this title. It could be a story about a girl in a manga that has some unique elements related to 'Tekking 101', perhaps a story about a girl who is into tekken - like fighting in a manga world.
Since the title implies a girl is central to the story, she's definitely a main character. There could be other characters like a mentor figure who guides her in the Tekking 101 world. Maybe there are some opponents she has to face in the story. For all we know, there could also be a group of friends she teams up with in the context of Tekking 101, but without more information, it's just speculation.
I'm not entirely sure specifically what 'garp save ace fanfiction' is about without more context. It could potentially be a fan - made story in which Garp tries to save Ace within the context of a particular fictional universe, perhaps related to One Piece.
Character development is key. Fanfictions explore how Ace and Luffy grow under Garp's influence. Maybe Luffy becomes more disciplined in some stories, or Ace learns to be more open - hearted, all thanks to their interactions with Garp and each other.