MC's contractor made a deal/offer to both MC (which is why MC can not really remember he dad despite loving him so much) and MC's brother (which is why he is an S-class). Contractor never made a deal/offer to the little sister.
MAJOR SPOILER BELOW as to why made an offer to the Bro and not the little sis
MC's Contactor has an obessive/interesting relationship with ONLY the MC...Originally MC and MC's bro where 1 being that later were split into 2 seperate bodies (which is why they are born in the same year). Due to this the Contractor has some interest in the bro but not the little sister. When MC was young (at dadś funeral) the Contractor made a deal/offer to the MC (which is why she doesn't really remember her dad despite loving him so much) and the bro (which is why he is S-rank). The contractor has no interest in anyone else (including the lil sis which is why he never made a deal/offer to her)
