Doubt this or not but he's one of Zeref Creations. Mard Geer is intelligent person but have an antagonist act. His Guild is Tataro's and he's pretty misanthrope. He don't really care whoever on his side.
Don't try to underestimate an guy like him. Even though he got turned into stone by the celestial king. (In the Manga, I believe Natsu killed him.) With his great thinking skills and that book. he's pretty strong.