There are four possible endings in these game, and different things you do throughout gameplay lead you to your ultimate ending. They range from Good to Bad, and here, I will discuss each ending, what happens, and how to get it. If you only get one ending and hate to have to play again to get the three others, rest assured, you're not missing much. In fact, the endings are all practically identical (save for one) with minor differences. Be warned, there are spoilers ahead.
Ending A: "Fortune Helps the Brave"
Fortes Fortuna Juvat The Good ending. In order to get this ending, you must defeat Debilitas during your battle using the chandelier, sparing his life. You must also have a great companionship with Hewie.

Nervous, Fiona fiddles with the locked door. The key is in the lock, but she is clearly frightened. Suddenly, Hewie whines and Fiona looks to him. She thanks him, potentially for calming her down and saving her life a hundred times, and then proceeds to open the door. As she does so, Hewie begins to bark. Fiona turns and sees the door open. It's Debilitas with a pair of hedgeclippers. There is a tense, silent moment where the two stare at each other. But then Fiona's face softens and the brute bows at her respectively. She leaves, calling for Hewie to follow her, and he goes about his business, trimming hedges in the empty castle.

Ending B: "Fire Tests Gold"
Ignis Aurum Probat The Okay ending. In order to get this ending, you must have killed Debilitas. To do this, either attack him repeatedly with Hewie or use combative items you find throughout the game. You must also treat Hewie moderately well. This is exactly like Ending A, except when Fiona looks back, she is alone. No one is there, just the immense, dark castle. Then she calls Hewie to leave with her. He darts ahead, ready to get the hell out of there, and she runs to keep up with him.
Ending C: "Grant Us Peace"
Dona Nobis Pacem The Okay ending. In order to get this ending, beat the game once, defeat Debilitas with the chandelier, then go visit him in his hut once the battle is over. He will give you a key, which ends up unlocking a key scene at the end of the game, if the key is used.

Again, it is the exact ending as Ending B, but after Fiona is long gone, you see Lorenzo calling out to her, begging her to stay for his own selfish reasons. His wheelchair falls down some stairs and he is left, alone, on the cold castle floor.

Ending D: "What you are, I was. What I am, you will be."
Tu Fui, Ego Eris The only real Bad ending, you must have played really terribly to get this ending. You must treat Hewie badly and not search for him in the forest. Therefore, he doesn't survive Riccardo's gun attack, so he cannot save Fiona, hence, he is dead.

Fiona wakes up in a glass box, where she is told by Riccardo that he has decided she will give birth to him. He tells her he needs he womb. She screams and tries to break free, but cannot. Later in the story, time has passed and Riccardo enters a room, the one during the beginning of the game where you can change costumes. He strokes Fiona's face and it's revealed she is heavily pregnant. He leaves and later, she wakes up. She smiles a little and begins to cackle. Clearly, she's gone mad. :C