Mermaid Melody Pichi Pichi Pitch/WMG

Sara is an avatar for the Dark Presence.
If you noticed Sara in her Face Heel Turn, her hair must have turned black because the Dark Presence has taken over her and plans to do what Sara wants: destroy the world. Thanks to her Love Redeems towards Mitsuki, Sara has control of herself again, and her hair is orange because the Dark Presence has left her.

The Big Bad is not Sara or Michel, but Satan.
Each character has demons to face:
 * Lucia can't tell Kaito that she's the mermaid who saved him.
 * Kaito teasing on Lucia.
 * Rina with her assholery early to Lucia and Hanon.
 * Caren has blaming Rina for not saving Noel, and her assholery to the other mermaids.

Sara and Michel are just plot devices to show what they become if they can't control their evil.

Taro Mitsuki is a Time Lord.
Maybe while in Germany, he's traveling back in time to see the good moments between him and Sara. Maybe he'll try to change the timeline of Sara's death, and that would explain of how did she appear in the end.

Taro Mitsuki is Friend.
He just lied about going to Germany. Instead, he leaves to Japan and causes destruction as a masked villain, all because he lost Sara to Gackto.

The series is All Just a Dream by Kaito.
Think about it. After losing his parents in a stormy night, he goes into coma, hoping to cope their deaths. The characters are aspects of himself.
 * Himself = Ego/Self
 * Lucia = Anima
 * Hanon = Super Ego
 * Rina = Id
 * Sara = Shadow Archetype

Rina and Caren are one and the same person.
If you noticed about the former when she is introduced, she is Aloof Ally to Lucia and Hanon. She's not one anymore as the series progress. However, this results with the creation of Caren, whom she represents the guilt of Rina not rescuing Noel and her aloofness earlier in the series.

The entire series is just a Dying Dream for Kaito.
During the flashback of what happened in the yacht as a kid, Kaito appears to have joined the victims of the tsunami. Even though Lucia manages to rescue him, he either never made it or died on arrival. From there, the entire series is his Dying Dream which encompasses his imaginings of what his life would be like when he came back from the trip. All of the mermaids are the elements of his subconscious;
 * Lucia is the first one to be described. She is the urged of to be liked by his peers.
 * Hanon is the sign of what would his life would have been if he hadn't acted like a jerk.
 * Caren is the manifestation whom Kaito, by this point, begins to realize of what the others see him as; an arrogant jerk whose only concern is doing things by his own. It would appear that her doings of sabotaging the mermaids' civilian lives such as interrupting Lucia's confession and leaving them a giant food bill are his reflections of his nature, not to mention the former is a sign of him being oblivious to the fact that she's the mermaid whom she met years ago. His own nature (Caren) gets captured, a sign of Kaito finally accepting his own ruthlessness and what an awful friend that he is.