From Heroes Wiki
Hiro brings Ando Takezo Kensei's helmet as a souvenir from the past.
Takezo Kensei wears his helmet when he first meets Hiro.
Hiro poses as Kensei, wears his helmet, and saves Yaeko from twelve attackers. Later, Yaeko asks Hiro to take off his mask, but Hiro tells her that he shouldn't. After stopping time and taking off his mask, Hiro disappears. When Hiro meets with Kensei again, Kensei is mad at Hiro because he stole his armor. Kensei takes back his helmet and armor, and is shot by arrows soon after.
After Kensei regenerates from arrow wounds, he wears his helmet and rides away on his horse. Hiro finds Kensei testing his powers while his helmet is on a fallen log.
Kensei puts on his helmet before he, Hiro, and Yaeko attack Whitebeard's camp.
Kensei wears his helmet as he confronts Hiro in a tent in Whitebeard's camp. They fight each other right before the camp explodes. Later, Hiro returns to the site, and sees Kensei's charred corpse with his helmet. Hiro takes Kensei's helmet and brings it to Ando as a memento.