Far from my favorites on the show, but I did mildly like the character and story.
During the first curse, he’s Dr. Whale, the leading medicine doctor in the town.
It was until after the first curse was broken that we learned he was Victor Frankenstein. Who technically jumped into the magical realm to save his brother or something.
Which led him making alliances/deals with the Mad Hatter, the Evil Queen, and Rumplestiltskin to get what he wanted.
After that, he’s more of a side character. He makes a cameo near the end of the series, where he seems to have taken up the trade of his original life.