Kuraimaru is Gohan's Clone.
After Cell was defeated,a small robot fly(possibly one from the Red Ribbon army)it gather data from Gohan and Cell's Battle.After returning to it's base,it relayed the data to it's creator.The doctor who made the fly(Dr.X) Created a clone in Gohan's image and with his all his powers,A year later the clone,broke free.He knew what he was,but he wants to kill Gohan, believing it his fault he is a clone.
Kuraimaru is a dark, cruel,cold, callous, dispassionate figure.Kuraimaru has absolutely zero remorse over his actions and can kill with out thinking twice.He also refers to him self with the suffix Sama (meaning lord or master)He shows no respect for living creates and believes all life is expendable.
The exact limit of his power is unknown however,it has been seen the he posses all for Gohan's abilities.(except Super Sayian and Dragon Buster)and some of his own,such as the ability to turn into a black mist,move thought shadows,summons shadow creates and weapons(mainly swords)rabid healing factor,and the ability to in evade others dreams(albet limited).He has also created a new version of Kamehameha(Ankamehameha) and Genki Dama(Angenki Dama).Kuraimaru has also created his own version of a Super Sayian(Due to the fact that he cannot become one) called "Shatter spirit mode".He has also been shown to absorb the powers and abilities of others thought touch of the head or neck area. Kuraimaru in also a genius,being able to rebuilt and recreate Dr.Gero's computer.
Kuraimaru looks,more or less,like Gohan,with the exception of his hair,which isn't spiky,but cut much shorter,He wears the same Gi as Goku,only with the Kanji for Darkness(闇 Yami)and dark purple. The sleeves are longer and black, as are his boots and shirt.His eyes are a dark purple,and his skin tone is a little lighter than Gohan.