One of the Witch Queen's (Ban Mido's grandmother) best disciples, Maria is an expert when it comes to magic and the occult. Outwardly, she resembles a busty lady in her late 20s (her bustline even beats that of Hevn's). But, it's actually just magic on her part; she's 99 years old and looks like an ancient mummy. The Witch Queen entrusted a young Ban to her, and she took care of him for a few years. She likens Ban to the son she never had, indicating the fondness she has of him. Although Ban was rude to her, he still has respect for his former guardian. She first offers her aid to the GB duo in the Divine Design (Kami no Kijitsu) arc. At times, she can act like a childish young lady; as such, she's quite fond of Ginji Amano, who turns into his chibi form around her.
She's been hinted to be romantically linked with Lucifer, at least in the past while the Witch Queen was still alive, but nothing came out of the relationship. Like Lucifer, she's the holder of the other set of the true Kami no Kijitsu.
As one of the most powerful member of the Witches' clan, she was feared and known by many. Her moniker, the Death Knell, demonstrated her power as the 'right arm' of the Witch Queen (possibly to counter Ban), nemesis for the Shamans. Her power was such that she, along with Ban and Lucifer, was able to revive Toshiki Uryuu, whose heart had been plucked out in the Divine Design world. This was maybe as an effect of her carrying the 'Stigma', as her powers, as remarked by Himiko, could even harm lesser witches and kill mere mortals in a glance.
In the "Voodoo Child" arc, she ran into difficulties during a fight at the Queen's Cup Challenge; she then suspects that a power greater than hers is at work here. In reality, this power belongs to her mistress's son - the man known now as Der Kaiser.
In the "Get Back the Lost Time" arc, she met up with the Kaiser....and realised that her mistress had kept some truths hidden even from her. After this meeting, she once again offers her advice and expertise to the GB duo, as they battle the Voodoo King and his minions.
However, after the Voodoo King was defeated by Ginji, she was to reveal a terrible truth: the GetBackers will have to face off against each other in a final "Ogre's Battle". Here, she reveals that she, too, is a bearer of the "Stigma".
Known as the "Death Knell" to shamans (the mortal enemies of witches), Maria's powers are not to be sniffed at. Her magic powers are powerful enough for her to defend herself most of the time. Granted, she doesn't fight much in the series. But, her ability to make her way through the Beltline to stand unscathed before Der Kaiser speaks volumes about her capabilities.
During the Voodoo Child arc, Maria displayed her prowess as a Witch. She used a repelling circle to protect herself from a Beltline citizen, HELP. However, in the end her magic was discounted by a complicated logic, which was explained by Akabane and Himiko. She was later saved by Ban.