Buster Keel!

"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, eg. Monster Users or Fighters. One young Adventurer, Keel, set off for his journey to find the adventurer, Shiva 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 Ravi, 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 Ravi might know where Shiva is...

(Source: MangaHelpers)