Ichiru is Zero's younger twin brother, and at a young age Ichiru adored Zero but after over hearing his parents talking about how Zero was stronger and sucessful vampire hunter, while Ichiru himself was weak.
When Ichiru later returns with Shizuka Hio, his misteress, he tells Zero why he chose this path and the "Curse of the Twins". As the plot thickens, Zero is slowly turning into a Level E vampire and then Ichiru comes to kill Zero, but it ends up being Zero who "devours" Ichiru, who i might add is stronger because he drank some of Shizuka Hio's blood.
ichiru is zero's identical twin... he chooses to be with shizuka than being with his family because he thinks that shizuka needs a companion and was envious of zero because he gets the attention of their parents...
he is zero twin brother.....they were Separated when ichiru Leave Hio kill his family and did nothing, and therefore Zero hate him soo much but in the end will ichiru close to death he help zero to kill rio .....