"I am behind you."
I remember reading somewhere that in some contexts kimi can imply an intimate/close relationship. It's very informal. When a classmate called me "kimi" in a skit for our final exam, I suggested that he leave it out unless he wanted to imply that I was his girlfriend. I'm a middle-aged woman probably older than his mother, so he turned red and then took it out. In any case, it wasn't an appropriate term to use with me.
When you talk to someone, use their name with the appropriate suffix such as -san or you can leave it out if it's obvious who you're talking to.
私は: "As for me" (The は particle marks "I" as the topic)
あなたの: "Your" (The の marks possessive for "you")
後ろに: "Behind location" (The に, here, marks a location)
あなたの後ろに: "Location behind you” (The possessive の combined with a に location means that object's location)
So, in crude grammar, the sentence can be read as "(As for) me, the location behind you (is where I) exist."
It's good to know that "anata" is not always polite, specifically when talking to people who are older than you or are somehow higher than you in the social hierarchy. I wouldn't worry too much about it as a learner of Japanese, though. Generally Japanese people don't get upset if a non-Japanese person doesn't use polite speech or calls someone higher than them "anata". I developed a phobia about using “anata” and it kept me from talking sometimes because I didn't know a person's name and didn't know how to address them.
I find it helpful to think of the の as indicating possession.
あなたの後ろ = your behind
後ろのあなた = behind's you
I think locations is a little confusing because the English is awkward, but when you look at two concrete nouns, the difference in placement becomes clearer.
猫のおもちゃ(neko no omocha) = cat's toy
おもちゃの猫 (omocha no neko) = toy's cat
" あなたの後ろにはいます " marked wrong. I'm hoping には is covered in the tips at some point further in the course. But please, would be great to have a quick explanation available here since I've seen には once or twice in the previous material and couldn't see the reason for its usage.
The most natural thing if you don't know someone's name is to avoid using a pronoun, but あなた would not be considered informal or impolite if you needed to specify the pronoun.
From a native speaker on HiNative:
Japanese use "あなた" when they talk to someone who they don't know their name.
For example, これはあなたのものですか？
If you know his or her name, you should not use "you".
For example, これは○○さんのものですか？