Soul Arms, otherwise called Reigen, are special weapons that Spirits can slowly learn to use. It takes experience, personal strength and immense will of heart -as well as a whole lot of time- to use Reigen properly. Because there are some that can use their Reigen and there are some others that can use their Reigen. That means that while eventually anyone might become able to draw out their Soul Arm, only a select few will become available to draw out its full strength.

Soul Arms consist of three basic materials. A kami, a container and a person's Reiki. The relation of those three is backwards. Reiki, when channelled into the container and depending on all the above conditions, may or may not awaken the Reigen's kami. The kami is the object's 'soul' so to speak and alongside the user's Reisatsu force the Reigen to take a certain form that fits that person the most.

Reigen is based on the 'open exchange' between the user's soul and the kami, so its much like a dialogue. You 'speak' to the Reigen and it answers by doing its own thing. This means that it is theoretically impossible for a person to use another person's Reigen. Unlocking a Soul Arm once it has been achieved requires the user to call it's name as well as an appropriate incantation to awaken it. Some Reigen are said to not need an incantation pronounced as it all depends on the kami's own personality -their names still need to be called, however, since you should always remember that names, as well as words in general, are power themselves.

The practice of Reigen began ages ago and started off with katana used as the spiritual containers which is what earned the Soul Arms their name. Nowadays other weapons or even pieces of armor may be used as containers for the kami. It varies according to the Spirit that is the user.

Reigen vary from normal weapons not only in that they correspond each to a specific Soul but also in that they posses powers beyond anything achievable by mere spells and those powers grow and develop the more familiar the Spirit's Essence becomes to them. If a certain level of sync is achieved an upgraded form of the Reigen becomes available to the user, even though they need to first release their Soul Arm to the first level, then take it to the next to avoid the strain on both their own soul and the kami -however level two release is also available at all times anyway, albeit with the consequences trailing after it.

Reigen-oriented powers need to be activated via incantations much like regular spells.

The people from DEUS are developing various devices that may -or may not- help enhance the Soul Arm, called UPGrades. Among them the most popular is the one-use-only Arm Tuner that raises the sync level of the person and Reigen for a brief period of time allowing attacks to escalate in strength while the effect lasts. Multiple use of UPGrades is not recommended, while extended use of them may result in the Reigen's degeneration due to the kami growing weak instead of stronger.