Discriminator rollers can be either round or square in shape, and are usually made of a different material than the rubberized / synthetic feed rollers. They are typically a softer material that can provide the necessary friction to prevent double-feeds.
Since it is in constant contact with the moving paper, the discriminator tends to wear down over time and become less effective (this will take many tens of thousands, or even hundreds of thousands, of scanned documents). Because of this, it is usually designed to be a user-replaceable part that is simple to access.
On some scanner models, the pressure of the discriminator roller can be adjusted manually, which can help with feed issues and/or compensate for normal wear and tear.