Takemichi learns several important lessons from his failures and mistakes throughout the series "Tokyo Revengers." Some of the key lessons include the importance of courage, determination, loyalty, and the value of protecting those he cares about. He also learns the significance of taking responsibility for his actions, not giving up in the face of adversity, and the power of making positive changes in the present to alter the future. Through his experiences, Takemichi grows as a person and becomes more determined to create a better future for himself and those around him.