The side character is the Regressor/Isekai, the protagonist is not.

The side character is the one who regresses or is involved in Isekai shenanigans, while the protagonist is simply trying to live their life in the world. Alternatively, a protagonist can be an Isekai character while the side character will be a Regressor (and vice-versa). Also includes series with Multiple Regressors/Regression shenanigans.

Second Chance from a Bad Ending
here (Fantasy Setting)
here (Modern Setting)

Protagonist aims to protect lead character from bad endings in romance/fantasy
here
Protagonist is reborn as a baby in fantasy/isekai
here

Multiple characters thrown to another world for isekai/fantasy
here

Additional (currently no MyAnimeList page):
-(Japanese Webtoon/2024) Yasagurete Shimatta Reijou wa, Shuuchaku Ouji ni Nabikitakunai! / The Rebellious Villainess Doesn't Want To Be Swayed By The Obsessive Prince!
-(Japanese Webtoon/2024) Ouji-sama no Konyaku Haki kara Toubou shimasu "Koko wa Otome Game no Sekai!" to Iiharu Seijo-sama to Happy End no Tame ni Te wo Kumimashita / Escaping from the Prince’s Broken Engagement
-(Manhwa/2021) For Stella (Alternative)
-(Manhwa/2022) My Sister's Happy Ending / I Am the Older Sister of the Possessed Female Lead
-(Manhwa/2023) My Sweet Enemy, Thy Name is Husband / Political Marriage With a Friendly Enemy (Multiple Regressors)
-(Manga/2022) Boku no Kawaii Konyakusha no Tamenaraba / For My Adorable Fiance's Sake.
-(Manga/2023) Okaa-sama no Iutoori!

Special Additions:
-(Korean Web Novel/2022) I Don’t Want to Be a Duke Anymore
-(Korean Web Novel/2022) I Became the Secret Mastermind in a Romance Fantasy Novel

Additional:
51-
A 'Part 2' will be created if this list goes up to 60.