Jacob is a minor character in "Chapter 4: The Laughing Merman" and it's tie-in novel adaptation "Hilda and the Laughing Merman".
In the animated series, he is never called by name, and in the book there's no image of him, nor is a description given of his appearance. It is thus hard to determine if Jacob is the Sparrow Scout with the hat, or the one with the long hair hanging in front of his face.
He is a member of the Sparrow Scouts and best friends with Harry. Both boys have no high opinion of Hilda, David and Frida because they always tend to get into weird adventures. They even dubbed the trio the "Freaky Friends". In the animated series, they first confront Hilda with this name as the Sparrow Scouts are preparing to go rafting but Hilda has trouble finding a fourth member for her group. In the tie-in book, they already confront her with their mean nickname during the bus ride to Sparrow Scout camp in The Great Forest. In both incarnations their teasing leads to Hilda's decision to try and have a normal Sparrow Scout camp for once without any adventures.
The boys appear again later in the story when Louise asks the merman Eugene to help play a prank on them for their bullying. Eugene uses his illusion of a sea serpent to frighten the boys. In the novelization, it is revealed that Jacob was so frightened by the beast he actually wets himself, though he fiercely tries to deny it. The boys leave Hilda and friends alone for the rest of the camp.