Here’s Ichigo’s Evolution from the very first episode to the moment he obtained the Final Getsuga Tenshou, drawn in Chibi form. We can see from the point he was a student he had gained his Shinigami powers, or had stolen Rukia Kuchiki’s Shinigami powers, which allows him to obtain such a large zanpakuto.
Ichigo was then faced with a decision, Rukia was chased down by her brother Byakuya and his Vice Captain Renji. Ichigo decided to get Rukia back by fighting Renji, although Ichigo had many injuries by Renji he was still had a lot of spiritual energy which allowed him to scare Renji, but was unfortunately defeated by Byakuya. He lost his Shinigami powers here.
Ichigo then started using his own Shinigami powers after he nearly turned into a Hollow during one of Urahara’s experiments. He also obtained a better zanpakuto. During the fight with Byakuya, Ichigo asked Byakuya to reveal his Bankai because Ichigo would surpass the Bankai of Byakuya using his own Bankai. Byakuya though he was joking but was surprised at the explosive energy and speed of Ichigo.
After some time, Ichigo was being haunted by his inner demon, Hichigo or Hollow Ichigo, he had to do something about this because he was slowly being taken over by his inner self. He contacted the Vizards (Visoreds) who helped him to control his power. In this form he defeated a great number of bad guys.
During the fight with Ulquiorra he had to call out his most powerful form from Hollow Ichigo’s help. Ichigo literally transformed into such a beast that Orihime and Uryu were both terrified, Ulquiorra was also shocked that a human would surpass his most powerful form. At this point a great character, Ulquiorra was defeated.
Ichigo had to train with his father Isshin in order to obtain the power to defeat Aizen. Tensa Zangetsu gave him the power to save his friends and family as well as defeat Aizen when he obtain the Final Getsuga Tenshou, which he used in the form of Mugetsu in order to lose all his Shinigami powers at the end.
Ichigo has gotten through a large number of forms, even now he’s changing his form into a more and more powerful being. You can see his latest forms in Ichigo’s Form Evolution – Fullbring to New Bankai Look.