As a child, Ichigo trained in martial arts so his body is in top shape for combat. He has always been agile and shown good coordination in his attacks.
He has Enhanced strength. Even when he’s not in his shinigami form, Ichigo still packs a punch.
He once smashed a man’s head through a concrete road with a single kick. He also has immense endurance and durability. The power his Bankai unleashes was strong enough to take on Aizen.
How did he get so strong?
There are primarily 2 things to consider here –
Ichigo’s father was a shinigami and his mother was a Quincy with a Hollow inside of her. So, when Ichigo was born he inherited the powers of a Shinigami, Quincy, and a Hollow.
The first Bankai form that Ichigo achieved turned his sword into a thinner black blade. This form let Ichigo use a stronger version of Getsuga Tenshou which he called Kuroi Getsuga. It lets a deadly blast of energy toward his enemies.
When he unlocked his Bankai he mastered some of the Hollow’s power that lay inside of him. As he gets stronger throughout the series, his Getsuga Tenshou upgrades into stronger attacks as well.
The Two Zangetsus
When Ichigo enters his soul to speak to his Zanpakuto spirit, there are two manifestations of it.
Old Man Zangetsu who has long curly hair and wears a long black coat is the manifestation of his Quincy side. Whilst “White Ichigo” is a manifestation of his Shinigami-Hollow powers.
For a long time Ichigo thought of “White Ichigo” as evil but later on, realized that hollow Zangetsu is also a part of him. Zangetsu or “White Ichigo” once stated that he represents Ichigo’s truest instincts.
Quincy Zangetsu and Hol Zlowangetsu are two halves of the same Zanpakutō. This also manifests in Ichigo having two swords. Ichigo only achieves his true potential by creating a balance between these two powers.
We find evidence of this when Ichigo was being trained by the Zangetsus. He lets the combined form of the two Zangetsus stab him which enabled Ichigo to master Mugetsu
Thus, it only makes sense why he has such immense power.