There's no way to know precisely without reading the story. But perhaps there is a scientist who studies the big wave, trying to predict its behavior. There could also be some local kids who get caught up in the events related to the big wave. And of course, there might be the rescue workers or sailors who play important roles in dealing with the situation that the big wave creates.
I'm not entirely sure as I don't have the story right in front of me, but usually there would be a character who is directly affected by the big wave, maybe a fisherman or someone living close to the sea.
Perhaps there are siblings in the 'the big wave children stories'. One older sibling could be more cautious about the big waves, while the younger one is more adventurous. There might also be their friends who join them in their wave - related adventures.
There might be some animal characters as well. Seagulls, for example, that are always around the big waves. They could be the children's friends, guiding them or giving them warnings when the big waves are coming. Dolphins too could be part of the story, playing with the children in the waves or helping them in times of need.
Jiya and Kino are the main characters. Jiya is a young boy who is initially afraid of the big wave. Kino is his friend who is more bold and tries to help Jiya face his fears.
Since I don't know the story, it could be the little wave itself as the main character.
Typically, there are students and a teacher. The teacher often plays a crucial role in starting the 'wave' experiment, and the students are the ones who get involved in it, with different personalities reacting in various ways to the situation.
It could be the sailors on a ship that encounters the rogue wave.
The main plot might be about a big wave, perhaps its impact on a community, or someone's experience with it. But without reading it specifically, it's hard to say more precisely.
I'm not entirely sure without reading the story specifically, but it could include fishermen as main characters since it's related to the sea and big waves. They would be the ones most affected by the big wave and its consequences.
I'm not entirely sure without reading it more closely, but perhaps there is a sailor. Sailors are often main characters in stories about rogue waves as they are out at sea where these waves occur.