I'm not sure specifically as there's not much information given. But typically in a visual novel, there could be a protagonist who is central to the hello - goodbye experiences.
Since it's a visual novel about 'hello goodbye', there might be a character who is very sociable and thus has a lot of hello - goodbye moments. Maybe a traveler or someone new to a town. They would be one of the main characters as their interactions with others would drive the story forward through these meetings and partings.
In a 'hello goodbye visual novel', there could be a pair of characters, perhaps lovers. Their relationship could be defined by their hellos and goodbyes. They meet, fall in love (hello), but then face challenges that lead to partings (goodbye). Their journey together and apart could form the core of the story, making them the main characters.