-ssi (씨) commonly follows a person’s name in polite social or workplace interaction, but it is not a universal “Mr./Ms.” equivalent. Using it downward, upward, or with only a family name can carry different nuances depending on the group.

-nim (님) adds respect and is often attached to roles or titles. In many professional situations, an established role title is more natural than forcing either -ssi or -nim onto a person’s name.

Local convention wins. Teams, schools, communities, and companies develop their own address habits. When you know the group uses titles, follow that pattern; when you do not know, a polite role-based or neutral opening is safer than guessing a familiar suffix.