Since they fight whenever they see each other, they did not interact much at the beginning and work on their own instead. However, when they constantly fail to catch the mermaid princesses (on top of the competition with the Black Beauty sisters later on), they realize they must cooperate with each other to win Gaito's affection. Therefore near the middle of the series (episode 21 to be exact, when all four entered the mermaid contest) we begin to see them being with each other and help each other out in times of need, especially when confronting the Black Beauty sisters. I think that besides the mermaid princesses, they are the ones who grow the most =)

One of the most remarkable traits about them is their absolute loyalty and unconditional love towards Gaito, although Gaito returns none for them. In the last episode, they sacrificed their own powers for Gaito, who was on the verge of defeat. This action caused them to change back to their fish forms...and even when Gaito's castle is sinking to the ocean floor they still desperately swim after it, to be with their love and master until death parts them.

After watching the series, I think this would be the perfect description for the Dark Lovers: Nobody is truly evil at heart and wish to harm others without a reason; the so called "bad people" in the world just have their own set of values...these values just happen to contradict societal norms. In the case of the Dark Lovers, we can tell that they are really not bad people (er...suiyou). For example, in the episode about them staying overnight at Pearl Piari, they get along very nicely with Luchia and them. Therefore they are only bad because Gaito is bad and wants them to do bad deeds.

THE DARK LOVERS - INDIVIDUAL PROFILES

IZUL The first of four Gaito's aides we encounter, who appeared in episode one. She is probably the oldest and most mature out of the four, judging from her semi-wrinkled face and her old-fashioned outfit ^^; In terms of personality, she is quiet and reserved, doesn't interact much with the other girls until later on in the series. Although she secretly is in love with Gaito and wishes to capture his heart, she is not as public about it as Eril and Yuri. She attacks by summoning fierce dragons made out of water, and whenever she hears the mermaid songs she suffers a heartache.

ERIL A "ghetto" kind of girl (or is she some kind of sea animal? I mean she has a pair of cat ears...O.o;) who is a bit on the insane side...no wonder, she has two personalities! She is "cute and hyper" most of the time, but when she changes while battling...her eyes develops a bright red glow, and her voices drops 3 octaves. She too loves Kaito-sama a lot. Her main attack is playing music that sound horrible to mermaids, which can easily be broken by the Pearl Voices' songs. When attacked (listening to "Legend of Mermaid"), she spins round and round while changing forms continuously ^^;

She can be a pretty fun suiyou when she is not on business. Whenever she sees something interesting she joins in with a cheerful smile, like in the episode when she helps Aori summon the rain, and actually succeeds! And she loves to show off her "instant schizo" skills to humans, who find that amazing. I can imagine her being a clown if she is not a suiyou xD

YURI She's a spoiled little brat, simple as that. Dressed in J-rockish medieval costumes, she thinks she's smarter than Izul and Eril, and brags about how they're so useless in capturing the mermaid princesses. But it's true though. She is the most powerful, and seems to be favoured by Gaito the most. I mean, he brings her along on his capture missions, and managed to catch Noelle. She plays the piano like Tarou, and can play both melodic music, and a song that makes humans lose their minds.

As the series progresses we eventually see a more gentler side of her. After Gaito yelled at her for failing to capture the mermaid princesses, she hides herself and cries in a little house by the beach, where human Hippo found her. Hippo not only puts bandages on her wound, he buys her a pair of sandals because she is shoeless. Since Hippo is the only person who cares about her, she falls in love with him. Her love remains even when she finds out he is with the mermaid princesses, and protects him no matter what. She probably has the most character growth because she definitely got more mature and much less bratty ^^

MARIA Gaito's "secret woman"! Her appearances are seldom, and she hardly interacts with Yuri, Eril, and Izul. However, she manages to settle their silly fights. She has access to a lot of places in the palace, including Gaito's grand bathroom. Think about the relationship between a company president and his private secretary ^_~ As in the words of Gaito, Maria is indeed a "cold" woman. She has the ability to summon hailstorms even on the extremely hot days, and anything she touches turns into ice.

...but that was only the beginning. Gaito eventually neglects Maria because of Sara's presence, so Maria tries to capture his heart the unethical way. She searches for a spell book in episode 44 that creates a super heavy snowstorm all of a sudden, covering the entire town in minutes. Once she approaches the Black Beauty Sisters for something, acting all subjective to them and all; I don';t know what her motive is, but my guess is that she wants to get close to Gaito through the Black Beauty Sisters, since they are more popular with Gaito than the Dark Lovers.