Soul Eater by Atsushi Ohkuba

Maka is training Soul to become a death scythe to be used by Lord Death.  The job was nearly done when they had to start all over.  Now they have to find 99 more souls before facing a witch to finish the training.

Black Star is the best ninja assassin in the world!  If only he didn't announce his every intention with a shout - making it hard for him and Tsubaki to take any souls at all.

Death the Kid is Lord Death's son.  Although he doesn't need to, he decides to start attending the Death Weapon Meister Academy with Maka, Soul, Black Star and Tsubaki.

Now they must all learn to work together -- for soon they will be facing the witch Medusa who is trying to make a kishin to destroy the world.

Soul Eater by Atsushi Ohkuba is a great manga series for both boys and girls.  It has action, the supernatural and lots of comedy.