Karacka

Karacka is a member of the Dark Guild Eisenwald.

Lullaby Arc
Karacka first appears when Erigor orders the Eisenwald guild members at the train station to dispose of the Fairy Tail team. Although most of the guild is promptly defeated at Erza's hands, he recognises her as Titania, the strongest woman in Fairy Tail, and hides himself in the walls using his merging Magic.

Later, when Erza and Gray leave to find Kageyama, Karacka is called from his hiding place by Byard. Byard orders him to prevent the escape of the Fairy Tail members by finding and killing Kageyama. However, after stabbing the man in the back through a wall, Karacka is visibly distressed, but still attempts a retreat. In a rage, Natsu charges the wall, destroying it, and seriously injuring Karacka in the process. It is assumed that Karacka was arrested along with the other Eisenwald guild members at the conclusion of the Lullaby Arc.

Fighting Festival arc
Karacka, who was released from jail at some point, watched the Fantasia Parade along with Kageyama.

Magic and Abilities
Caster Magic User: Though it remains unnamed, Karacka's Magic allows him to merge with, and travel through solid obstacles. This technique has been used to hide effectively, and also perform sneak attacks, such as his attempted assassination of his partner Kageyama.

Trivia

 * In the manga he stabbed Kageyama in the back with a knife but in the anime it appeares that he punched him or went into his body with his hand. This is due that to the fact that the anime does not depict blood.