Since I don't know the specific 'isekai koushoku musou roku novel', I can't say for sure who the main characters are. But in many isekai novels, there is often a protagonist who is transported to the new world, so perhaps there is a character like that in this novel as well.
I'm not entirely sure as I haven't read this novel specifically. But generally, 'isekai' often involves a character being transported to another world. Maybe this novel combines that with elements of 'koushoku' which might refer to certain character traits or story aspects, and'musou' could imply a sense of adventure or power. Without reading it, it's hard to say for sure.
Since I haven't read the novel specifically, I can't say for sure. But it's likely to have the reincarnated sage as a main character. There may also be characters who help or oppose the sage in their journey.
I'm not sure specifically as there could be many different things in such a novel. Generally, in an isekai (another world) type of novel, it might be about a sage (kenja) who is reincarnated (tensei) and embarks on a journey of becoming extremely powerful (musou).
I'm afraid I can't give a definite answer about 'hp1 kara hajimeru isekai musou toc novel' without having read it. However, generally in isekai novels, the story often begins with the protagonist being transported to another world. Given the 'hp1' here, perhaps the character starts in a very vulnerable state. And'musou' usually implies some sort of epic battles. So it could be about a character's struggle and growth in an isekai world starting from a weak state with a focus on battle - related content.
I'm not sure specifically as I haven't read it in detail. But generally, in a light novel with the theme of isekai (another world), it might be about the adventures of the characters along with a 'damegami' (a term that could refer to a certain type of deity or character) in the other world, perhaps involving battles and power - ups as'musou' (unrivaled) implies.
I'm not sure where you can specifically find this 'hp1 kara hajimeru isekai musou toc novel'. You could try looking in local bookstores that carry a wide range of Japanese novels (assuming it's a Japanese novel).
Since we don't have a lot of information about 'Roku the Christmas Story', the main characters could vary greatly. If it's a story set in a village, there could be a village elder who gives Roku advice on how to celebrate Christmas. There might also be some children in the story who look up to Roku and join in on Roku's Christmas adventures. Maybe there's a reclusive old man that Roku befriends and helps to find the joy of Christmas again.
Some similar light novels could be 'Overlord'. It also has a strong isekai theme with the main character being very powerful in the new world. Another one might be 'That Time I Got Reincarnated as a Slime'. It involves a character being reborn in another world and gradually becoming very powerful.