You could start from the chapter where the anime ended. Usually, there are forums or fan communities that discuss this and can give you specific chapter recommendations.
One option could be to check out popular online manga libraries such as KissManga or MangaFox. Just make sure to look for the official and authorized versions. Also, some bookstores might have physical copies or offer digital downloads if you prefer that.
There are likely other important characters as well. For instance, characters like Dazai Osamu who is known for his intelligence and rather mysterious nature. His actions and decisions play a significant role in the story's development. And then there might be characters who are antagonists, who oppose the main characters and create conflict.
I don't know the exact main characters as I haven't read it. But they could be the most prominent beast - like figures in the story. Maybe there's a heroic beast character who drives the plot forward.
Most BSD light novels often have complex characters. Since they are based on literary figures, they bring in the depth and complexity of those original personas. For example, a character might have the same traits as a famous writer but with a unique twist for the BSD universe.