Ellie is Earth's partner.

Although small and young, Ellie is wise beyond her years and is capable of great strategies while Earth knows of many of the legends of the Mamodo World. Earth and Ellie's relationship is notably similar to that between a master and servant (at Earth's request).

(Source: Gash Bell Wiki)

Kiyo is a brilliant junior high student whose inflated ego (and tendency to blow the grading curve) has made him a major target for teasing at school. So his father sends him a bizarre birthday present – a strange little boy named Zatch Bell – to help him make friends and reform his bad attitude. Zatch brings with him a mysterious red volume of spells, and Kiyo discovers that Zatch has magic powers that are unleashed by reading from the book! But there are more surprises to come, and Kiyo's destiny is about to change forever! 
 
(Source: Viz Media) 
 
Note: The chapter count includes the volume-exclusive bonus chapter “The Two Queens”.

Takamine Kiyomaro, a depressed don't-care-about-the-world guy, was suddenly given a little demon named Gash Bell to take care of. Little does he know that Gash is embroiled into an intense fight to see who is the ruler of the demon world. All of the demons have to pick a master on Earth and duke it out with other demons until one survives. Needless to say, Kiyomaro becomes Gash's master, and through their many battles, Kiyomaro learns the importance of friendship and courage. 

[Wiki (jp)](https://ja.wikipedia.org/wiki/吉田小南美) | [Wiki (en)](https://en.wikipedia.org/wiki/Konami_Yoshida)