The VF-31J, (used exclusively by Delta Squadron) was developed as a general-purpose support variable fighter exclusively for the Tactical Sound Unit 'Walkure'. Though the basic airframe and construction are descended from the VF-31 Kairos, internally the addition of a 'Fold Wave System' and different engine necessitated numerous internal changes and reinforcements in order to accommodate the improved engine performance. The VF-31 Siegfried currently has C, E, F, J, and S variants fielded, with the base specs being relatively similar, primarily differing in external appearance - such as the head section design and number of beam guns installed there. (The J model has one anti-air beam gun while E, F, and J have two, and the S model has four.) Though there are essentially no significant internal differences, each model has a number of software changes, with the fold wave system on each model being tuned to the individual parameters of each Walkure member, and matching the the ARIEL III onboard AI with the movement patterns of each member, in addition to minor avionics changes.
The S model, as the commander unit, has formation command systems as well as improved transmitters and datalink systems.