–Male Diclonius when he meets Lucy
Lucy's Half-Brother was a manga-only character. Male Diclonii are almost unheard of and/or extremely rare in the narrative.
In the manga
Lucy's half-brother is the only true male Diclonius seen or spoken of in the Elfen Lied series. He was born through the rape of Lucy's long-lost mother by Chief Kakuzawa. He was a vital part of Kakuzawa's plan to kickstart the new Diclonius race, as Kakuzawa aimed to breed his son and Lucy and place himself as the "god" of the new race. Like the viable clones of Mariko clones, the boy had a mind-control device implanted inside his head, which kept him under control and inhibited his ability to speak.
Towards the end of the series, Lucy was taken back to Kakuzawa's Island HQ and held in a secret underground grotto, where she was told of his plans and introduced to her half-brother, who seemed to recognize their familial connection. Rejecting Chief Kakuzawa's megalomaniacal agenda, Lucy decapitated both him and her half-brother. She then apologized to his headless corpse by saying that neither of them had a right to exist, that their bloodline must end, and that she would also die very soon. This boy was also the half-brother of Professor and Anna Kakuzawa. Despite the Kakuzawa family's racial beliefs, this boy was the only member of their family who had ever been a true Diclonius.
Whether it was due to the mind control implant or his genuine feelings, he seemed very happy to see Lucy and eagerly tried to call out to her, albeit with childish pronunciation. It's not known if Kakuzawa told him about his plans for him or if the boy could even understand them if told about them, considering he was a child, but regardless, he could have recognized Lucy as his sister using the same psychic ability Diclonii use to recognize other members of their species.
The time-frame of his conception and birth is difficult to determine. He looks to be almost ten years old, but Lucy was only captured approximately four years prior to her one and only meeting with her half-brother, and if Chief Kakuzawa's claim is to be believed, Lucy's mother was only found out about after Lucy's capture. Even if the Chief (and by extension, Lynn Okamoto) had meant the mother was captured as soon as Lucy's existence was surmised, this would only add a few to several months to his age. Even with an immediate identification and capture of Lucy's mother, almost another year would have passed before the Male Diclonius was born, making him at best three during the final confrontation. Since he was not a Silpelit, he, like Lucy, should have aged normally, and not at an accelerated rate. One possibility is that Chief Kakuzawa knew of Lucy and her mother long before her existence was even postulated by Kurama and Professor Kakuzawa. Given the Chief's secretive nature and obsession with his family's supposed Diclonius heritage, it can be speculated he was on the lookout for the birth of horned children outside his family. However, nothing in the series confirms or supports this. Another time-frame problem is that the Chief seemed set to have the pair mate soon, raising questions once again about the boy's age and whether or not he was near puberty.