There may also be characters who represent different social strata or ideologies in the story. Some could be poor and marginalized, while others might be wealthy and powerful. Their reactions to Vishnu's death could be influenced by their social positions, and they could be main characters in terms of driving the narrative forward by showing different perspectives on the event.
One possible main theme could be the exploration of mortality. The very title 'The Death of Vishnu' implies a focus on the end of life, perhaps examining how different characters in the novel react to the impending death of Vishnu, and through that, exploring broader human attitudes towards death and the afterlife.
In addition to the above, in some versions of the Vishnu love story, Hanuman also has a connection. Hanuman's devotion to Rama (an avatar of Vishnu) is a form of love in a way. He helps Rama in his quest for Sita. So, Hanuman can be considered as a character related to the Vishnu love story, albeit in a different capacity compared to the more direct love - interest characters like Sita and Radha.
The main character is Lord Vishnu of course. His consorts like Lakshmi also play important roles. And then there are the various demons that he has to defeat in his different avatars, such as Ravana in the Ram avatar story.
One main event is the story of Vishnu's avatar, Rama. Rama's exile, Sita's abduction by Ravana, and Rama's victory over Ravana to rescue Sita are important. Another is Krishna's life events. His birth, his interactions with the Pandavas and Kauravas, and his role as a charioteer in the Kurukshetra war are key.
The Vishnu original story is centered around his role as a supreme being in Hinduism. Vishnu is considered to be eternal and omnipresent. His stories are often intertwined with the cycle of creation, preservation, and destruction. In the original stories, Vishnu sleeps on the cosmic ocean, and from his navel, a lotus grows, and from that lotus, Brahma, the creator, is born. Vishnu's various avatars play crucial roles in different epochs. They come to fight against the forces of evil and protect the good. For instance, the avatar of Matsya (the fish) saved the first man, Manu, from a great flood, which is an important part of the Vishnu lore.
I don't know the specific 'City of Death Novel' you're referring to, so I can't say for sure. But it could have a brave hero who is trying to save the city from whatever is causing all the death.
I'm not sure specifically as I haven't read the novel, but likely there would be a detective or an investigator type of character who is trying to solve the mystery of the death.
I'm not sure without reading the book, but often in a novel set in Death Valley, there could be a rugged explorer as a main character.
The main characters are likely the Death Troopers themselves. They play a central role as the novel is named after them.
Since I haven't read the 'ambassadors of death novel', I can't say for sure who the main characters are.