Elfen Lied

Synopsis

Kurama's past is explained in this episode. He was a good friend of Professor Kazukawa, who invited Kurama to work with him and his father on the diclonius project after leaving university. He reluctantly agrees, however, as he is performing research, he is infected with the diclonius virus via transmission through their vectors, and as a result, his daughter Mariko is born a diclonius. He decides to kill her, however as his wife dies from complications after giving birth, begging him to spare her, Kurama chooses to let her live under Kazukawa's condition that bombs must be planted inside her to kill her in case she turns aggressive. Meanwhile, as both Lucy and Nana are now part of Kohta's household, the facility decides to send "the most powerful" diclonius, known only as "No. 35".

(Source: Wikipedia)