@MKJ Technically yes, but would you really answer that question so vaguely? That's a question usually answered with a name. You could answer serveral questions with just "she is" but the asker will probably ask for more information. Orynae was right in saying you wouldn't use it that often. We don't use such a simple sentence that often in English, either.
In English, we do happen to use terms such as "he/she did." Also, if we're talking to a group of people, a lot of the times we run into the situation where, the person talked about being part of the group and, hence, nearby, we receive a reply like "She/he is" accompanied by the pointing of the index finger toward that person nearby. I mean regardless of how un-often it is used, whether it is used or not is my inquiry because to say that "it isn't used often" means that, albeit rare, the sentence or term is correct and that's what we care to know here.
I'm a native English speaker so you're preaching to the choir here. I did answer your question in the most basic terms, as you desired. "Technically yes." Your original example of "Who is here? " wasn't the best because someone only asks that if they cannot see who is here, hence why I said "she is" wouldn't suffice. Your second example includes pointing to the person in question. In that situation, yes, obviously you can just say "she is" because the person asking can see. The point I was trying to make was that language is situational and contextual. Regardless if you can use simple sentences, you won't unless it is appropriate for the situation, as you demonstrated with the pointing example.
If you put a noun after "the," like "The girl eats," then yes that would be correct. Essen is an irregular verb, just like "to be" in English (assuming you're a native English speaker here). Subject-verb agreement is different for every subject. When I first learned German, I got so frustrated with there not being a way to tell which verbs are irregular. Unfortunately you can't know until you learn the word. If anything about grammar gives you trouble, and this goes for everyone who might read this, I highly recommend buying a Schaum's grammar workbook from Barnes & Noble or Amazon. They are tough but fantastic, and I find physically writing things rather than typing helps with memorization. Sometimes it pays to do things with pen and paper!