Imu’s first form = Base Luffy
Imu’s second form = Luffy Gear 4
Imu’s third form = Luffy Gear 5
This must be the awakening of his fruit, given that the trees around him are affected by his power.
I believe Imu likely has at least two more forms: a Zoan form related to Mariejoa, and a final, awakened hybrid form.