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).
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.
Yes. I can recommend two doujinshi novels to you: "Three Kingdoms: Cao Cao's Eldest Son Assassinated Dong for His Father" and "Time Travel to Orochi Musou." The former was the story of crossing the Three Kingdoms and becoming the eldest son of Cao Cao, obtaining the opportunity to become a Peerless Battle General, while the latter was the story of the protagonist buying a controller and crossing into the Orochi Peerless, fighting alongside historical figures. I hope you like this fairy's recommendation. Muah ~😗
The 'Roku Christmas Story' might be a unique, fictional account. It could involve Roku as a character in a Christmas - themed narrative. For example, Roku could be a little elf in Santa's workshop whose job is to make sure all the Roku - powered devices in the world are ready to stream Christmas carols and holiday specials. Or it could be about a person who gets a Roku as a Christmas gift and how that changes their holiday entertainment experience.