He gradually expanded his holdings in the center of Tosa province. In Eiroku 3 (1560) Kunichika made a surprise attack on Nagahama castle held by a rival clan, the Motoyama. When word reached Motoyama Shigetoki, he was greatly angered and gathered 2,500 troops from Asakura castle in order to take Nagahama back. Kunichika left Okô castle to intercept Shigetoki’s army with 1,000 men. This was Motochika’s first battle. He was 23 years old. This was a very late first battle for a son of a Sengoku warrior, however there was a reason for this.
Motochika was born with a pale complexion and a meek face, and had a quiet, reserved demeanor. Because of this, his father’s rough warrior vassals referred derisively to him as "the little princess". Kunichika also felt that Motochika was an effeminate child, and so waited to bring him to the battlefield. When Motochika left Okô castle on the way to the battle, he asked his vassal Jinzenji Bungo for tips on how to use a spear and how to lead an army. Motochika joined Kunichika’s army with 20 mounted men.
At Nagahama, both armies collided. A long and bloody battle ensued. Finally, both armies pulled back momentarily to regroup. At this time Kunichika remembered his son’s participation, asking about his location on the battlefield, but was unable to find him until a vassal pointed him out in the forest shade about 200 meters from the left wing of the army formation. Motochika was gathered there with his 20 horsemen. It appeared that they had not even fought yet. Kunichika was recorded as referring to them as "lazy fools".
At this time, Motochika’s isolated group was noticed by a force of 100 mounted Motoyama warriors. With a battle cry, the 100 Motoyama warriors charged. It did not appear that Motochika’s small force would stand a chance against them. However, Motochika’s battle cry was heard across the battlefield, "Attack! Attack!" Motochika spurred his horse, charging out ahead of the rest of his small force into the oncoming rush of horsemen, wielding his spear with great skill and speed, fighting with the ferocity of a wild lion. The 20 men with him were encouraged, and fought a hard and bloody battle. Kunichika stood in his stirrups, yelling for attack. The Chosokabe army responded by surging forward. The Chosokabe army, strengthened by the surprising effort in battle of the "little princess", was unstoppable. The Motoyama troops were completely crushed, fleeing back as far as the main encampment. The pursuers swept away everything in their path, easily crushing the headquarters. Motoyama Shigetoki fled to Urado castle, barely escaping with his life, and presumably Chosokabe Motochika was never called a "little princess" again.