kaa san

from my understanding, Okaa-san is Mother, Onee-San is Big Sister and Ojii-san is Grandfather, now while Kaa-san and Nee-san i can still determine refer to Mother and Sister i'm assuming Jii-san is something close to Grandfather as Rein in Katahane is almost like a grandfather to Cero and since Wakaba is close to Cero (potential love interest . the gym san diego