What’s a person who borrows something but won’t return it?

I cannot find the right words or expression or idiom to describe a person who borrows something from someone but will not return it to its owner.

Also, I would like to know the word that describes a person who no matter how many times you tell them to give you back the money, they just smile and do nothing.

The word "deadbeat" is defined by Merriam-Webster as "one who persistently fails to pay personal debts or expenses", though other dictionaries specify that this usage is particular to North America.

As for someone who "smiles and does nothing" when someone demands payment of these debts, I suppose one might call that a shameless deadbeat.

I think of the word leech. It refers to FPT sites. Several years ago these sites would often have ratios where people would get a certain amount of downloadable bytes for every byte they uploaded. Sites that did not have ratios were said to be "leech" sites because you could download all you wanted without having to upload.

The term now more refers to any individual who is often into the piracy scene and doesn't do anything but download files for cracked versions of software, movies, music and so on.

They don't share what they have downloaded and don't provide help to others looking for certain files. I guess that might help you

Here are some ideas (although I like 'Dagwood's neighbor Herb' very much).

Scofflaw -- someone who shamelessly ignores his financial and other legal obligations; this only intersects partially with what you're looking for, I think.

Scrounger -- a person who borrows from or lives off others.

Moocher or mooch -- from to mooch, to borrow a small item or amount without intending to return or repay it.

Schnorrer (Yiddish) -- I'm not sure if this matches up well. The definitions I've found on the internet don't seem to, however, the same word in German works great.

(If things get really bad, you could say parasite -- but that is very strong.)