Webfrom his story, we understand his reserved attitude and his difficulty being around the other hashiras.
Since he lost his friend, who died protecting him, giyu feels.
Webgiyu suffered from imposter syndrome and survivor's guilt after losing his sister and his friend sabito.
Giyu's past trauma and guilt humanize him, paralleling tanjiro's journey.
Webwith sabito dead, giyu has been carrying this grief with him the whole time.
He feels inferior, and believes that it's why he won't manifest his demon slayer mark.