I'm a little new to this game and don't know everything yet, but I have found that there are several jutsu which hurt your enemy for more damage, increase your chance to hit, and decrease your chance to be hit. Now one thing I would just like to suggest is the idea that, make it a super rare event that requires the defeat of that creature. A.K.A you beat the creature but cannot kill it, and in the last moments seal it inside yourself (much like the Third Hokage and Orochimaru's arms). Now this fight would be extremely difficult, as the beast has more power, and the beast you fight is random among the nine. But, due to the fact that you are such a weak vessel, you can only seal a portion of the beast inside of you. Being a demon vessel gives you one power, the ability to say that I killed so and so beast. Its a thing of prestige.
Also, you could make their abilities present, such as using 9-tails form at the cost of 75% HP, to gain a 5x attack bonus or to use a special ability (such as the Kyubi chakra aura) in order to gain a higher hit percentage for a single attack (more than a normal jutsu if there is one)... these differences would be slight, and the biggest part would be to say that I have killed the 9-tail fox!
P.S. if you read the Naruto series closely there are portions in which you are lead to believe that Naruto may only have a portion of the fox within him, and that the 4th Hokage took the main of the best, but due to overflow Naruto had to take some of it. Another thing this may be, is that each time you use the "super technique" (with small stat increases during a fight) you lose HP each round or can even die, due to the stress on your body, which means that this would be a technique that comes with great risk.
P.S.S. this would be great if it was PvP only, since it would be such a rare occurence that you could get such a small advantage on someone that it would help you in PvP, but when it comes to normal fights it dosn't do squat...
If anything we should just get it so we can have it in our statistics
Hell you could even have at the beginning when you register "A demon was sealed inside of you... what demon was it again?"... It would put some diversity to the game. They don't even have to do anything really.