Its about soul eating! Woo lol.
So you can be a technician, which is basically the controller of the weapon, or you can be a living weapon. The weapons are in human form, but morph into the weapons letting the technicians use them. The souls(or wavelengths) of the technician and the weapon must be in sync though, or they cannot work with each other.
Main Characters:

This is Soul Eater, and Maka. Soul being the boy, Maka being the girl. Soul being the weapon, the scythe. Maka is the Technician. Their signature sync is the Witch hunter, which is a powerful strike in which the scythe grows a parallel side and grows 2x bigger, gaining an eye and striking a blue wave of energy.

This is Black Star and his weapon, Tsubaki, who can turn into a chain scythe, giant shuriken, smoke bomb, knife, and a sword. As you can tell from her abilities, Star is a ninja. His signature sync is a giant chain star which can trap an enemy and act as a shield.

Finally, this is Death the Kid, with his two partner weapons that can turn into twin pistols, and if they do their signature wavelength sync, the Death cannons, which are as they seem..giant cannons that shoot hella strong beams. On the left side is Patti Thompson, and in the middle would be Liz Thompson.
Great anime. Heres a fight between Black Star and Death the Kid to get you interested.
A good fight with Black Star using Tsubaki and Maka using Soul if you're interested in the weapons.
Death the Kid using his pistols.