The background in Luo Xiaohei's War Chronicles is in a parallel world with various monsters, animals, plants and humans.
Agen is Luo Xiaobai’s country cousin, and he is recognized by the demons as the ancient mythical beast Xuanli (but he denied that he is Xuanli in episodes 21 and 22. As of October 2019, Agen and Xuanli’s relationship is unknown). Can absorb fire, control the form of water and condense it into ice (the abilities and characteristics are the same as Xuanli).
Extended information:
Plot synopsis of "Luo Xiaohei's War":
A little black cat living on the street was brought home by the girl Luo Xiaobai , named Luo Xiaohei. Luo Xiaohei is not an ordinary cat. He is very humane. He can squat on the toilet, refuse to eat cat food, and his long tail can even split into multiple organisms called "Hei Xiu".
On the other hand, a mysterious figure named "True Ting" mobilized his three winged wolves to search for Luo Xiaohei's whereabouts. Soon after, Luo Xiaobai took Xiao Hei to the countryside to visit his cousin A Gen and his grandfather, and various bizarre and fantasy events occurred.
Baidu Encyclopedia—Luo Xiaohei’s War