Sidney Glass was a pretty notable minor villian for the short time he had on the series. I still remember him mostly well.
In the original land, he was Regina’s magic mirror. He served the Evil Queen well.
But he was originally a genie who worked for Snow White’s father. Regina tricks him into thinking she loves him and he eventually becomes a cursed magic mirror.
In Storybrooke, he is some kind of journalist that works at Regina’s propaganda machine. She tries to use him to be the candidate for Sheriff so Emma Swan doesn’t get it.
For which he fails, but he pulls a trick on Emma thinking they were working together to bring Regina down.
He mainly spends the rest of the series in the mental ward for taking the fall for Regina’s deep dark crimes. Which is a little odd I think, because everyone should have known the truth after the Curse was broken.