I was reading that Sugimoto only had a bit part in the Twelve Kingdoms books, and Asano didn't even exist. Is there any explanation as to why they were added to the anime? Sugimoto has a pretty big role in the anime, but Asano felt pretty pointless. Were they just added to give Youko somebody to talk to?
It's important to know that in something like manga, even though thoughts are able to be shown, there is still a very heavy visual element for the reader. In a novel, like The Twelve Kingdoms, it is conveyed entirely through text and we are at the mercy of the characters' inner thoughts much of the time.
Asano was added largely as a confidant (and a childhood friend) of Youko; he is someone who could externalize the struggles that Youko has internally in the novel. Sugimoto, like Asano and even Yuka, fills a very similar purpose. Her role is described as a foil* for Youko, helping her to externalize issues that the viewer would not see (but would be able to read in the novel).
Asano also adds some serious depth to the story (some serious spoiler material here):
The gradual loss of his mind and his inevitable death are very powerful, mostly because they follow his desperate want to find a purpose in his life. And where better for him to die than in Shoukei's arms?
* In literature, theatre/theater, etc, a character who helps emphasize the traits of the main character.