Five tails is actually a pretty strong taiiled beast. It is arguably the fastest(or second fastest next to Kurama). It has boil release that allowed Han to have incredibly fast and dangerous taijutsu. It also has incredible physical strength.
As the Jinchuriki of Kokuo, it is very polite so with mutual respect, it shouldn't cause many problems constantly.
But since the MC will be either Han or before him, he would have to fight in atleast the 3rd Great Shinobi War. Use its abilities only for defence.
The MC must be a honorable person to match Kokuo's personality.
He/she can develop lightning release to work in combination with Steam and boiling water.
Developing lightning release will give an advantage against Iwa, in case he has to defect to get away from war.
The main goal should be to survive Akatsuki.
Include interactions with Roshi and Son Goku.
If the MC defects, a secluded forest-like area will be good with large water body in surrounding.
MC can try to get some sort of sage mode in seclusion and let Kokuo gather Nature energy for him/her. Maybe Horse or Dolphin summons?
MC can also save some people like Karin and her mother and bring them with her, and train Karin. Another person to bring with can be Guren.
MC should be either able to kill Pain, or should be able to be uncaptured until last, only losing some chakra to Akatsuki like Gyuki. Maybe Kokuo's horn?