Let's start with the first one we meet in a very scary and disgusting opening scene.
Debilitas is a failed attempt by Riccardo to create a homonculus, a humanoid created with alchemy. He is mentally disabled man with the mental capacity of a child. He acts as a handyman and gardener for the castle, and considering how lovely all the gardens are, he clearly does a really good job. He is seen as a villain, but really, he is just a mislead man. Debilitas has a love for dolls, and to him, Fiona is a doll. She is afraid of him and doesn't understand his intentions, which sometime seem to border a little perverse and sexually curious, but we never really find out. There are, however, many sexual undertones throughout a playthrough of the game.
After the cat and mouse game Fiona and Debilitas have through most of the game, one final confrontation in the chapel stops him from pursuing her. In fact, he seems to have found a respect for her, like she is some kind of saint. It's sad to say, at the end of the game when Fiona leaves, we can only assume Debilitas spends the rest of his lonely days in that castle where everyone is by then dead.
Daniella is a succesful attempt by Lorenzo to create the perfect woman. She is, in fact, pretty close, as far as looks go. She is another homonculus in the castle that acts as a maid and cook for Lorenzo, and probably for Debilitas as well. Daniella has a dangerous fixation with herself and perfection. She is insanely jealous of Fiona for being young, beautiful, and most of all: human. Her personality borderlines schizophrenic. She will go from wanting to kill Fiona to practically ignoring her.
Daniella wishes she could experience all the pains and pleasures of being a human, specifically sex and child-bearing. However naive and innocent Fiona may be to this, Daniella knows that in her shadow, she is imperfect. Her hope is to become human by killing Fiona and taking her uterus. In the end, after all that Daniella hated Fiona, it is because of Fiona that she got to experience one human emotion: pain. When she is impaled, Daniella thanks her and for once in her life, is happy. It also happens to be the last moment.
Riccardo Belli, like Fiona's father, Ugo, is a clone of Lorenzo. The difference between he and Ugo is that Ugo received Azoth and Riccardo did not. This leads to jealousy and obsession. He believes he needs Fiona's Azoth passed down to her to reach immortality, hence his constantly pursuing her (and in the bad ending, ultimately impregnating her with himself). In addition to that, if he gains his immortality, it takes away Lorenzo's chance of doing the same.
At the castle before Fiona's appearance, Riccardo is a groundskeeper. He wishes he could be the one of power giving the orders, and we know this based on how he claims to Fiona that the castle is his. He is the master and leader. This is not the case as he takes orders from Lorenzo. This all makes him a very angry, jealous man who will do absolutely anything to gain his immortality and feel powerful. He is like a hollow shell of a human, completely soulless.
Pictured to the left is a young, corrupt Lorenzo, not the wheelchair-bound decrepit old man we are used to in the game. He is a deceitful man whose sole intention is to gain immortality, whatever the cost. He misleads Fiona by making her think he is an ally, helping her through most of the game. In fact, technically, he is Fiona's grandfather, considering he created his clones, Riccardo and Ugo. Lorenzo is also a perverse old man as he peeks in on Fiona while she changes clothing. He is clearly happy to do this.
He is the worst of them all because were it not for him, the castle would not be full of such evil, soulless individuals who would kill to get what they want.