I'm not entirely sure without reading the novel, but likely the main character is the veterinarian. There may also be some interesting side characters like the locals in the other world who help or hinder the veterinarian's work.
The main character is, of course, the veterinarian who gets transported or somehow ends up in another world. There could be a mentor - like figure in the other world who teaches the veterinarian about the local flora and fauna. And perhaps there are some patients, which could be strange and wonderful creatures, that play important roles in the story as well.
Since it's a 'Veterinarian in Another World' web novel, the central character is the veterinarian. In addition, there might be a local healer or wizard who initially doubts the veterinarian's methods but later becomes an ally. There could also be some animal companions or magical beasts that form a bond with the veterinarian and are crucial to the plot, each with their own personalities and abilities.
Well, the obvious main character is the veterinarian. This person is likely to have a distinct personality, maybe they are very kind - hearted and determined to help all creatures. There may be some important magical beast characters as well. These could be a majestic dragon with an ailment that the veterinarian has to cure, which then becomes an ally. Also, there could be a local healer or shaman who at first is skeptical of the veterinarian's ways but later becomes a friend. And there might be a character from the veterinarian's original world who somehow also ends up in this other world and they have to figure out how to get back together.
Well, of course, the veterinarian themselves is a central character. They are usually highly skilled in their field, with a love for animals. Then there are often the magical or fantastical creatures that the veterinarian treats. These can range from talking animals to huge, mythical beasts.
Well, in 'A Veterinarian in Another World' web novel, perhaps the unique part is how it combines the veterinary profession with the elements of a fantasy world. For example, the animals or creatures in that world might have special abilities or characteristics that make treating them different from normal animals on Earth. This would create a lot of challenges and adventures for the main character, which is quite engaging.
There's Touya Mochizuki, of course. Then there are the various female characters who play important roles in the story. For instance, Yumina Urnea Belfast, a princess who becomes an important part of Touya's life in the other world. There are also other characters like Leen, a magician, and Elze Silhoueska, a swordswoman, who all interact with Touya and contribute to the story.
There may be other supporting characters as well, such as suppliers who provide the ingredients for the restaurant. They might be normal humans or have some connection to the otherworldly elements. Their presence adds more depth to the story by showing how the restaurant functions in both the normal and otherworldly contexts.
The veterinarian could have all sorts of wild adventures. Maybe they encounter magical creatures and have to figure out how to treat their unique ailments.
Since it's about a king's oath in another world, there could be a character who is an antagonist, someone who opposes the king's oath for their own gain. Maybe there's a wizard or a powerful noble. And of course, the king's friends or comrades, who could be from different backgrounds like a commoner with special skills or an elf who has knowledge of the other world's magic, would be important main characters too.
I'm not entirely sure who the main characters are without reading the novel in detail. But typically in such a story, there would be a protagonist who is the cook in the other world.
Since I haven't read the exact web novel in full detail, it's hard to say precisely. However, based on the general concept, there would be a male lead who is thrust into this other world and the labyrinth. He would be the main character around whom the harem is built. There might also be female characters who have different personalities and abilities, and they gradually become part of the harem as they interact with the male lead.
The main character is likely the one who goes to the other world with Smartphen. We don't know much more without reading it, but they are probably the central figure through whose eyes we experience the story.