Alright I think I got the hang of it
Possessing Spirit is the real ultimate technique of the Niko Style, Gaoh's real long term plan was finding and training a suitable student and later transfering his soul to the young body, literally the "tiger's vessel"
However, not every of the Nikos could handle the strain of possessing spirit and a lot died, including our Niko's friend. Niko, being an idealist of martial arts, was disgusted and betrayed the other Nikos to form his own branch of the style and find Gaoh to stop his plan once and for all. On the meanwhile, the Nikos still loyal to Gaoh did their fair share of fucked up shit to find potential vessels like the Gu ritual, explaining Agito being visibly pissed when thinking of "Niko"
Some time pass and our Niko, who has settled down as a bodyguard, finds Ohma and decides to train him with his own style, including Niko's own ultimate technique, Demonsbane. However, one of the loyal Nikos (Tigerman) was already on his trail, and tried to hijack the training to turn Ohma into a vessel, eventually leading to our Niko to put the shackles and get killed. But the other Nikos are still running around, whether or not they all stayed loyal to Gaoh, still doing their evil deeds, and will be the antagonists of Part 2
All of this to say
The inside, not even once