Jigorou Kuwajima

Jigorou Kuwajima was the man who trained Agatsuma Zenitsu and Kaigaku. He was formerly a Pillar in the Demon Killing Corps.

Jigorou mainly appears in the manga through the forms of flashbacks. He was a physically short and elderly man with silver or grey colored hair and a visible scar located below his left eye. Jigorou often carried a wooden cane with him likely to support his body due to age. He is commonly seen in the manga wearing a jinbei styled kimono similar to the one worn by Sakonji Urokodai.

Jigorou had a steadfast devotion to his students and strict training. Despite his status as a strong mentor and experience as a previous pillar, he had a soft spot and unwavering dedication towards raising the next generation of students.

Zenitsu noted that despite how often he whined and ran away, Jigorou never gave up on him once. He had saved Zenitsu from a lifetime of debt and taken him on as a student. Showing unyielding patience, he was happy just to see Zenitsu master the first form. He had a great belief in training skills to perfect through repitition comparing it to forging and purifying the steel of their swords.

(Source: Kimetsu no Yaiba Wiki)