Translation:Who is that person?
Yes in English it is mostly about physical position. "This" thing is closer to you or the one you just picked up to show someone, and "that" thing is something you have to point to because it's over there. However, "that" can also be the implied distance of disliking something. If the thing is right next to you but you don't like it or didn't choose it you would say I don't like "that." And if two men are standing right in front of you, you would say I like "this" guy and not "that" guy. Therefore, that can mean physically further away or something you wish was physically further away because you don't like it.
个 is a measure word used for people (as well as certain other nouns). This means that if you want to specify a number or use this or that to talk about a person, you must put the measure word inbetween them.
You should not say 这人，you should say 这个人. You should not say 三姐姐，you should say 三个姐姐。