Carmilla is an Immortal, and later revealed to be an acquaintance of Sabata. After her purification near the end of Boktai: The Sun is in Your Hand, her soul is sealed away in Sabata's body, just like Otenko's. In Boktai 3: Sabata's Counterattack, her soul is finally released, after being mentioned by Sabata a number of times in Boktai 2: Solar Boy Django. She is first seen as a young girl, but transforms into a creature resembling a snake-like banshee afterwards. She petrifies Otenko, who is presumed dead. She is fought twice in the first game. During the second fight, she is referred to as "Soulless Carmilla". Before she died, Carmilla was known as "the crying witch", because she was shunned due to her powers. The Count saw this, and took advantage of her, turning her into a banshee Immortal. Sabata carried Carmilla's soul until Boktai 3, where it was released from his body and began to help Django to free Sabata from the influence of Ratatosk.
Carmilla -- in her "human" form -- uttered Sabata's name after her defeat in Dark castle. Sabata's links with Carmilla lead some players to believe that the two were once in love. This is supported by her quote at the end of Boktai 3 when she teases Sabata; "Don't you love me anymore?". Ironically some players link Carmilla's appearance to Lita, however, unlike Lita, Carmilla's eyes appear more sorrowful and her hair is darker and different in style. This is also the case with Django and Sabata. Despite these links there is no known relation between Lita and Carmilla.
Carmilla returns in Lunar Knights as Professor Sheridan's maid and is strongly implied to be Lucians lover Ellen after being turned into an android by Stoker. Carmilla in Lunar Knights has not been confirmed to be her, but both Carmilla's bear a strong resemblance, including the spot below Carmilla's right eye.
This might be a reference to one of her quotes at the end of Boktai 3 to somewhere in the cycle of life and death to meet again with Sabata.