Buster Keel!

Buster Keel! (バスターキール!, Buster Keel!) is a manga series written and illustrated by SAKAMOTO Kenshirou. It has been running in Kodansha's Monthly Shonen Rival since October 4, 2008, ending with its 47th chapter in July 2012.

Plot
"Adventurers" are the group of people who set off on the adventure to defeat monsters of different ranks. The adventurers are divided into many categories according to their fighting style, e.g. Monster Users or Fighters. One young Adventurer, Keel, set off for his journey to find the adventurer, Siva the Monster User, who is called as the world's best for defeating the fearsome S-ranked monster known as Dragon Ape and who he claimed to have taken something very important away from him. He met with Lavie, another adventurer who is a Monster User, with her monster, Mippy, and decide to stick with her because he was told by the person in the town that Lavie might know where Siva is...