He is a Shinso who attempted to kill off the Human race in seven days. By absorbing numerous Ayashi and gaining their powers, he became a huge monstrous entity without equal, but it also cost him his human form. However, he made a clone of himself before he was sealed away by Akasha 200 years ago, and the clone subsequently assumed two identities: the Masked, who created Fairy Tale and the Miao Family, Miyabi Fujisaki, while using the alias in present day.
Alucard has thus far displayed traits more akin to an aimless beast of little or no intelligence, however he is noted for having two driving forces in his personality. Firstly, for a yet unknown reason, Alucard detests humanity, such that he became a monster and then tried to wipe them out 200 years ago. Secondly, due to his 200 year slumber and the sheer hunger he is suffering from because of it, Alucard is a voracious and self-indulgent eater who sees everything around him at the moment of awakening as mere food. This includes other monsters, his "supposed" allies, his own granddaughter, Akua, (although it is not clear as to whether he even knows who she is), and basically anything that breathes.
It is assumed that Alucard's mind is indeed still intact, hinting that he does have intelligence, in any event, his ravenous hunger and sheer cruelty still make him little beyond a monster.
Alucard is known to be the oldest vampire in existence, and may have been the most powerful monster of all time, as he is shown to have a powerful aura long after his death. It was only the powers of the Three Dark Lords that kept him from destroying mankind. Even now his power is held in high opinion because only amazingly strong or well informed people know him even in death people seek his vast strength. It is believed that his mind remains intact even after all these years.
After Akua attempts to kill Akasha Bloodriver, Moka's Shinso power, which she inherited through the blood given to her by her mother at birth, awakened, giving her incredible power, but also waking up Alucard in the process. Alucard's tentacles encircle the Shuzen's mansion, grabbing any living thing within its reach and sprouting smaller eel-like creatures that drain the victim of blood to satiate Alucard's centuries-old hunger.
In chapter 42 it is revealed that Alucard now resides in the Fairy Tail Headquarters and Moka has been put in a special seal next to him to break the Rosario seal and free Inner Moka, allowing their blood to synchronize and awaken him. However, Outer Moka is willing his revival time to be delayed at the cost of herself. However, Moka was eventually forced to awaken by the sight of Akua supposedly killing Mizore and Kurumu, thus waking Alucard as well.
As Alucard's awakening transpired, he consumed countless Fairy Tale monsters to satiate his hunger, just as he'd attempted to do 7 years ago, and altered the landscape of the Floating Garden with his devastating search. Gyokuro later arrived and stole Moka's rosario, thus placing Alucard under her control.