You can improve your character by winning fights. You will unlock new moves by learning them in the dojo moves.
You can also use gems and coins to unlock gear and weapons, such as knives and swords to upgrade your character.
Coins
Coins can be earned from winning battles. Farm quick matches to earn coins easily. Use a hard hitter style that lets you earn bonus coins.
Play Survival mode to earn coins. The more rival ninjas you defeat in a round, the more coins you will earn.
Tournament mode is a good way to earn coins. Fight in the tournaments and take on any demonic henchmen.
Defense
Master evasive moves such as taking a knock when you exit a backflip. If you are in a situation where your opponent keeps pressuring you, giving yourself some distance.
Gems
Complete achievements to get free gems as well as other rewards.
Level up to get a free gem and coins.
Infinite energy
After you are out of energy, set the time of your device forward by thirty minutes. Then, set the time back to the correct value. You will have full energy.
Various Gameplay Tips
If you're having problems with a particular stage go for a duel or survival round and play some in order to get coins. Then use those coins to upgrade your weapons, armor, and helmet and then go for the troublesome stage. You can only duel once every four hours so just forward the time on your phone in the time settings and you'll get your dueling privileges again. Body armor should be first on your shopping list as it will increase both your attack power and your defense.
Character improvement...