I'm not sure specifically as it depends on the particular fanfic. But they could be young hippies, musicians, or artists who were part of the Summer of Love scene.
Obviously, Ladybug (Marinette) and Chat Noir (Adrien) would be the main characters. Their dynamic relationship as superheroes and the potential for a summer love story between them is really exciting.
Nikki is likely one of the main characters. She often takes center stage in Love Nikki stories.
The main characters are Tom and Summer.
I'm not sure specifically as it depends on the version of the story. It could be two young lovers, perhaps named Jack and Rose, who find their love growing under the Summer Triangle.
Sometimes, new and younger characters also take the spotlight in these fanfics. For example, Ms. Marvel might join the Avengers during their summer adventures. Her fresh perspective and new powers can add a different dynamic to the group. Also, characters like Doctor Strange might be involved if there are magical elements in the story, which could be related to some summer - themed mystery or threat.
There's no way to tell without reading the fanfic. It could be a crossover fanfic, in which case it might combine characters from different works. Or it could focus on minor characters from the original work and give them a chance to shine during this 'summer like no other'.
Since I haven't read it, I'm not sure specifically. But they could be young lovers, old couples, or people from different walks of life who experience love during summer.
Given the title 'Alex Morph Summer Love Story', Alex is most likely the central character. As for the others, if there is a love story, there should be at least one more character, perhaps a friend or a stranger that Alex meets during the summer and eventually falls in love with. It could also be that there are some supporting characters who play a role in the events related to Alex's transformation (morph) and the development of the love story.
I don't know the exact main characters as I haven't read the novel. But they are probably some young girls who are at the center of the love stories in the book.
It could also focus on some of the minor characters from the original series. For example, Neville could be a main character in this fanfic and his growth and change during the summer could be the main focus. Or Luna, with her unique perspective, could be in the center of all the happenings during that summer of change.