@Miyono-chan The honorifics "-ssi" and "-nim" basically have the same usage. It is meant to address someone formally as respect.
However, a person usually use -ssi if he or she is in equal terms with the speaker like acquaintances and such. As for -nim, we usually use this honorific if a person is of higher status than them or someone really unfamiliar to you.
Hana calls V with honorific -nim cause he is head of RFA, while she calls everyone with honorific -ssi since they are all members of RFA.