Training outside of regular class is an important part of the martial arts, both with friends and other students, but also by oneself- practicing the kihon of the martial arts- ukemi, kamae, san shin no kata, etc.
But what about the various kata from the ryu that transmit the feeling of the Bujinkan- both found in the jin ryaku no maki section of training, and the schools themselves?
Can they be practiced solo?
Unlike some other martial arts, the kata found in the Bujinkan dojo are practiced with uke and tori- two or perhaps more people. They are tools to not only transmit and convey the movement and philosophy of soke, but also vehicles to understand distance, balance, timing, and rhythm.
They are a way to interact with your every changing training partners and make the concept work- learning how to adapt.
That part can’t be done just practicing solo.
Even some of the kata which tend to have more of a striking flavor, such as those found in Koto ryu require a partner.
This is why to *really* get the transmission it needs to be done in person, felt, and understood.
BUT, they can be done to help review the steps, and initial part of movement, working on a general proficiency of the various parts that make them up- and some of them do have quite a few steps.
These is some value and proficiency in solo/alone kata training, but that is not the primary method of them in our taijutsu practice.