–The Twins, providing the answer to one riddle and the start of many others
The Twins are a pair of manga-only characters who make their debut in the final chapter.
In the manga
For many years after Lucy's death, Kouta would go to wait at the grave of her puppy for her, but she never appeared any of those times. Several years after the birth of his daughter Nyuu, named for his deceased friend, he's waiting at the grave with Nyuu by his side when two young girls obscured in shadow approach them. The girls are apparently friends of Nyuu Jr., as she mentions playing with them to her father before they arrive and calls out to one of them as she realizes they're there. Since they are well-dressed and don't appear to be malnourished, it's highly likely they are well taken care of by whoever makes up their family.
Implied is that these two girls are Lucy and Nyu reincarnated. Supporting this is the fact that one of them is named Kaede, their hairstyle is similar to Lucy's when she was a child, and their hair ribbons are done up in the same style as Nyu's once were. They are also waiting at the spot where Lucy told Kouta to wait for her so many years ago, and one of them says she and her sister are there to wait for "a very important friend." Nyuu Jr.'s dialogue even implies the girls live in the area and play with her near the gravesite often.
Whether or not the girls are human or diclonius is unknown due to their hair ribbons obscuring whatever horns they might or might not have. They appear to be as old as Nyuu Jr., making it possible they were born after the birth ban, so if they are indeed Diclonii, the chances are slim. If they were Diclonii, they must have been born into a loving and understanding family due to their seemingly healthy condition. Whether the girls are Lucy and Nyu's reincarnations and whether they're human or not are among the last mysteries left when Elfen Lied ends.
Unlike Lucy, written as 楓 in Japanese, the twin Kaede has her name written as カエデ, using katakana symbols.
Since there is no concrete proof as to whether the girls are human or diclonius, they will have both tags for the sake of categorization in this wiki.
Since only one twin is named, a possible name for the other twin is Momiji (which can be written as 紅葉 ,もみじ , or モミジ), since it also means "Maple" referring to the Japanese Maple. Other twins with this naming scheme are in the anime Blue Seed.