You are either a Weapon Meister or a Demon Weapon, and have a partner of whichever type you aren't. The Demon Weapon can be of any type seen in the series besides a Death Scythe, or of a type of your creation of roughly equivalent power. Your soul resonance is very compatible with each other to start, and you both have experience similar to that of Maka and Soul at the beginning of Soul Eater in both the specialized abilities of Meisters and Weapons as well as in the use of the fighting styles associated with the type of Weapon you chose.
Weapon Meisters specialize in manipulating their soul wavelengths to produce various techniques. With training, the Meister of your pairing may learn to use this to sense souls, to strike with their own soul and to adjust their wavelengths on the fly to negate soul-based forms of offense, among other more specialized techniques.
Demon Weapons are humans with the ability to transform partially or fully into a specific weapon, though certain types may have multiple weapon forms. They also can eat souls and act as a conduit to amplify the soul wavelengths of a Meister. At the start, the Weapon of your pairing may only do broad changes to their body such as transforming a limb into the blade of their weapon form or popping their torso out of the blade when otherwise fully transformed, but with training they may perform more delicate changes.
The more synchronized the two of you are, the more your abilities are augmented when the Meister is wielding the Weapon, and through this you have access to a powerful combo ability, such as Witch Hunt Slash or Death Cannon, which if used well can potentially kill even immortal beings, but this ability is draining and hard to use.
If one of you dies, the other can house the soul within them; if a new body is constructed for the dead, they may inhabit it.