Awaken is the most important thing to power up a character. Once the character reaches its max level, it can be awakened to increase the star level and stats. Additionally, some of the characters acquire secret techniques, which are special moves requiring more chakra than Ninjutsu, as being awakened.
What are needed to awaken?
Awakening requires specific awakening materials by characters. For example, Naruto Uzumaki ( The Worst Loser ) at star level 3 needs Awakening scrolls ( Heart Book ) to be awakened.
It also costs Ryo, which was a currency of Japan in the past. A player can earn Ryo by succeeding missions and as reward for deceiving conditions during missions.
You can awaken a character only once. If the character was initially bronze it can be awakened to silver and similarly from silver to gold and from gold to blue. If the character was initially bronze you cannot awaken it to gold.
Your Sasuke is not capable of becoming a 5-star, and the I'm guessing it says level 70 rather than max is because it's possible to limit-break it, increasing the amount of levels of your character from a max of 70 to 75, and so on and so forth.