Since it's called 'Echoes of the New World Story', it might have a character who is like a guide or mentor. This person could have some knowledge about the new world and help the other characters navigate through it. There could also be a character who is resistant to the changes the new world brings, adding some conflict to the story.
Some possible main themes could be discovery, as it might be about exploring the new world. Another could be change, since new worlds often bring about a lot of changes for the characters involved.
Since not much is known about Half Life Echoes Story specifically, it's hard to say for sure who the main characters are. But if it follows the pattern of the other Half - Life games, there could be a scientist - type character who is trying to make sense of the strange events. There might also be a resistance fighter who is leading the charge against the alien or Combine threats.
Often in these types of novels, there is a family member who is central to the story. For example, a mother or a father figure. They could be the one that the stir of echoes is centered around, or they could be trying to protect their family from the strange and often dangerous events that the stir of echoes brings about.
One of the main characters is a mage - like figure. They possess powerful magic skills which are crucial in the battles against the dark forces in the story. Another main character could be a warrior type, strong and skilled in combat, protecting the group during dangerous encounters.
I don't have the exact information on the main characters yet. But typically in Final Fantasy games, there are brave warriors, mages, and other characters with unique skills.
I don't know for sure, but they could be ordinary people thrust into extraordinary situations in this new world.
Since we don't know the specific 'new world story', it could be anyone. It might be a young adventurer, a scientist, or a group of survivors.
There might be a best friend character. This friend could play an important role, either as a confidant for the protagonist's unrequited love woes or as someone who tries to help the protagonist move on. For instance, the friend could introduce the protagonist to new people or give advice on how to deal with the situation.
There could be a character who is haunted by the past. Since the title has 'echoes of time', this character might constantly be reminded of events from their earlier days. And in relation to the river, perhaps there is a character who works on or near the river, like a fisherman or a ferryman. They could be key characters in the story, interacting with others and being part of the plot that unfolds around the river and the concept of time.
I don't know the main characters in 'echoes of eden graphic novel' as I haven't read it. You could try looking for reviews or summaries online that might mention the main characters.