The basic knowledge you need to know before watching the anime is that the setting takes place in modern day society, but the premise is that there is essentially a divergent breed of humans, the "diclonius", or two-horned humans. The diclonius possess telekinetic "hands" known as "vectors" that possess incredible strength and has the edge to cut through almost anything, including human flesh. Although the diclonius may seem cruel at first, they have multiple personalities; one being the sadistic murderers, and the other being a mentally scarred innocent child. All the meanwhile, the government that's basically run by a large private company (corruption in society) studies research on the diclonius to determine the danger they pose, given their tendency towards mass homicide. Humans take the worst aspects of both halves if the diclonius, presenting conspiracy-level power and lack of accountability, while running without any oversight as long as they get results. Throughout the anime, the diclonius are treated abysmally, and are not even given the basic considerations due as rational beings.
Elfen Lied is a dark and tragic story about a girl named Lucy (aka Nyuu), her caretaker Kouta, and his cousin Yuka. We start off with Lucy, the main character of the series who was initially held in a facility built for experimentation, located off the coast of Kamakura, but she managed to escape and wreak havoc. During all that chaos though, Lucy got injured which caused Lucy to develop a secondary, childlike personality also known as Nyu (Nyu's innocent personality developed --> Lucy's violent personality forgotten). Kouta and his cousin Yuka find Lucy's body washed up on the beach shore at night, and decide to take her back to their home and help her out (this was one of the negligible things I had a problem with in the anime, because nobody that finds a stranger naked with blood stains and with horns will just take them to their house, without calling the police or something, this was really random). Fortunately for Kouta and Yuka, when Lucy awakens, it's in her clueless and innocent Nyu form. But the government is not willing to let Lucy go that easily, they'll try to catch her no matter what kind of repulsive, disgusting method they have to use.
There is a lot of death, betrayal, and murder off all ages that shows the disgusting nature of humans. This anime is exceptionally deep, delving into topics such as human nature, dehumanization, racism, child abuse, ostracism, bullying, inequity, redemption, forgiveness and love. It dances around these themes and interweaves them ingeniously, creating an anime that is both beautifully deep, and horribly poignant.
"What does it mean to be human? Is there ever equity in a society? Can we alter our character? Are we wise enough to know what changes to make?
There is the look at the nature of racism, and how it leads to individuals being singled out and ostracized, or even leading to outright abuse which causes a person's behavior to change (psychology). They say that person's behavior and attitude is affected by that person's surrounding environment, and in Lucy's case, it surely did. When Lucy was young, just because she had little horns on her head, everyone bullied her, ostracized her, and did inhumane things to her. What does it mean to be human? Just because you were born a human doesn't make you a human; you have to be humane to be a human. Why do humans think that we're greater beings than any other species? It's because we're the most arrogant, selfish, repulsive yet wondrous creatures alive. It's suggested throughout the anime that things would have turned out quite differently had Lucy simply been treated with the love due to a child.
As a counterpoint to Lucy, there is also the character Mayu, who is completely human yet she is present to support the theme. Mayu was the victim of neglect on the side of her mother, and sexual abuse the side of her father-in-law. She shows that abuse is not limited to the dicloni race, but is a problem even without racial overtones in human society.
As for the characters, all the main characters were wondrously developed with deep, dark, well explained pasts. Lucy is personally my favorite female character in all anime. Why? Because everything about Lucy was simply amazing, enjoyable, sinister, and she’s given great character development, a tragic backstory, and a personality with a surprising amount of depth. You'll have to watch it to understand but the anime does an amazing job turning a ruthless killer into the protagonist that you cry for and have no choice but to love. There are other diclonis but I would have to spoil part of the plot to reveal things about them, so I'll mention that they all play a symbolic role in the big picture scheme of the anime and are once again are very "organizedly" developed. Kouta and Mayu also are marvelously developed with short but detailed flashes of their past. Also sometimes Kouta and Mayu can be annoying, they're only human, they're bound to be scared, weak, and careless when having to deal with the ugly dark side of our society today. I must say, Kouta is really annoying.
Personally I enjoyed Elfen Lied so much, that I'd give the enjoyment aspect of it a 10/10. I personally love looking deep into psychological and philosophical matters, and that was there this anime shined. I was truly enthralled and captivated by all the universal concepts and themes that the plot had to throw at us. Of course if you don't care much for deep psychological/philosophical thinking, then you may disagree with me. The action scenes are short, but intense and heart-breaking. Nudity is an aspect that many people criticize Elfen Lied for, and of course there is nudity in it, but the actual sexual overtones are few and far between. For me, this struck the perfect balance, though for those who are more squeamish and can't think beyond just what's happening, this is definitely not an anime for you.
Elfen Lied is very creepy and gory, and the well done animation is part of the reason why it is pulled off so nicely. The nudity is NOT ever for fan-service which is good. The amount of blood can also get a bit excessive, but considering the level of dismemberment, it isn't hard to believe that there would be that much blood. The character designs are generally pretty distinct, and the lighting for a given scene is virtually always looking proper. There are also color imagery symbolism to look out for (absolutely genius). As for the sound of the anime, I'll start by saying that personally I believe that the dubbed version of Elfen Lied is utter shit, but the subbed version is a great. The voices match each character's personality and appearance. The opening of Elfen Lied, Lillium is one of the saddest, omniscient and profound openings/songs ever. The other soundtracks were also a tragedy to listen to, not in the sense that the music was bad, but in the sense that they were so beautiful. There so many soothing, yet dark soundtracks in Elfen Lied, and it fit in perfectly with the story. Unfortunately, they got too repetitive... The opening Lillium was played in every episode multiple times...
If you haven't already seen this anime, I'd recommend you watch it and instead of watching it for pure enjoyment, I hope you actually see beyond the screen into the big picture, because that's what Elfen Lied was made for. This anime is a tragic and deep story, although I wouldn't acclaim it a "masterpiece" for obvious flaws and lack of "entertainment". Honestly, if you're looking for entertainment, look elsewhere.
