so in class we read the Tale Tell Heart story, and i’m assigned to be the narrator’s attorney. i have to introduce the narrator, but they don’t mention their name in the story. how do i introduce the narrator while being assigned to defend him because of the murder he committed?

Well, i guess you could be on his side and say something like "The narrator couldn't control what he was doing because of the eye and the sound of the mans heart beating in his ears". This is what drove him to committing the murder along with confessing it as well.  

The defendant or the narrator

Explanation:

You are the narrator's attorney therefore he is the defendant. So you can call him the defendant. You can also call him the narrator since the Tell Tale Heart doesn't state his name.

what are the lines. come on!

specific audience will understand the main ideas. for example, if you were writing an instruction manual for rocket scientists, it would likely look very different from one you would write for young children. your language, tone, and writing style would change significantly.