Takemichi learns several important lessons from his failures and mistakes throughout the series "Tokyo Revengers". Some of the key lessons he learns include the importance of taking action to protect those he cares about, the significance of standing up for what is right even in difficult situations, and the value of never giving up despite facing numerous challenges and setbacks. Takemichi also learns the impact of his decisions on the people around him and the power of second chances to make a difference in changing the future.