It's hard to say which is more popular in an absolute sense. The anime has a wider reach in terms of visual media consumers. It can attract those who are more into watching shows rather than reading. However, the light novel has a dedicated fan base among readers who prefer the in - depth exploration of the story that only written works can offer.
Issei Hyoudou is the central character in Highschool DxD. He's a typical horny high schooler at first, but his character evolves throughout the story. Rias Gremory is a significant character as she's the one who gives Issei a new life as a devil. Then there's Akeno Himejima, who has a complex personality and is quite powerful. Koneko Toujou is also important; she has her own backstory and plays an important role in Issei's journey. There are also other characters like Xenovia and Irina who contribute to the story in different ways.
The 'Highschool DxD' light novel and its anime adaptation follow the story of Issei Hyoudou. He is a perverted high school student who gets killed on his first date. However, he is reincarnated as a devil by Rias Gremory. From there, he gets involved in the complex world of devils, angels, and fallen angels. He has to fight battles, make alliances, and also deal with his newfound powers and the harem of girls around him.
Well, the anime often has more visual and auditory impact. It can bring the story to life with cool animations and voice - acting. However, the light novel usually goes into more depth in terms of character thoughts and some detailed plot developments that might be skipped or simplified in the anime. For example, in the light novel, there could be more in - depth exploration of the main character's inner struggle about his new life as a devil, which might be just briefly shown in the anime.
The anime left off at a point that was somewhat mid-way through the novel series. But it varies depending on how you define the specific plot points and story arcs.