The side character is the Regressor/Isekai, the protagonist is not.
Manga
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 in Fantasy Setting
https://myanimelist.net/stacks/36912
Second Chance from a Bad Ending in Modern Setting
https://myanimelist.net/stacks/55404
Protagonist aims to protect lead character from bad endings in romance/fantasy
https://myanimelist.net/stacks/37175
Protagonist is reborn as a baby in fantasy/isekai
https://myanimelist.net/stacks/37356
Multiple characters thrown to another world for isekai/fantasy
https://myanimelist.net/stacks/37924
Additional (currently no MyAnimeList page):
-(Manhwa) My Sweet Enemy, Thy Name is Husband / Political Marriage With a Friendly Enemy (Multiple Regressors)
-(Manhwa) For Stella (Alternative)
-(Manhwa) My Sister's Happy Ending / I Am the Older Sister of the Possessed Female Lead
-(Manga) Boku no Kawaii Konyakusha no Tamenaraba / For My Adorable Fiance's Sake.
-(Manga) Okaa-sama no Iutoori!
Second Chance from a Bad Ending in Fantasy Setting
https://myanimelist.net/stacks/36912
Second Chance from a Bad Ending in Modern Setting
https://myanimelist.net/stacks/55404
Protagonist aims to protect lead character from bad endings in romance/fantasy
https://myanimelist.net/stacks/37175
Protagonist is reborn as a baby in fantasy/isekai
https://myanimelist.net/stacks/37356
Multiple characters thrown to another world for isekai/fantasy
https://myanimelist.net/stacks/37924
Additional (currently no MyAnimeList page):
-(Manhwa) My Sweet Enemy, Thy Name is Husband / Political Marriage With a Friendly Enemy (Multiple Regressors)
-(Manhwa) For Stella (Alternative)
-(Manhwa) My Sister's Happy Ending / I Am the Older Sister of the Possessed Female Lead
-(Manga) Boku no Kawaii Konyakusha no Tamenaraba / For My Adorable Fiance's Sake.
-(Manga) Okaa-sama no Iutoori!
Manga, ? vol, 2014
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Author:-
The alternative, there is a Reincarnator and an Isekai character.
Manhwa, ? vol, 2021
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Author:7
The alternative, there is a Regressor and an Isekai character.
Manhwa, ? vol, 2021
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Author:8
The alternative, there is a Regressor and an Isekai character.
Manga, ? vol, 2021
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Author:7
Manhwa, ? vol, 2022
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Author:7
Added as a special exception due to character circumstances.
Manhwa, ? vol, 2021
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Author:9
The alternative, there is a Regressor and an Isekai character.
Manga, ? vol, 2019
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Author:8
Added as a special exception due to multiple POV tag.
Manga, ? vol, 2021
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Author:-
Manga, ? vol, 2020
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Author:-
Manga, 3 vol, 2022
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Author:-
Multiple Regressors
Manhwa, ? vol, 2020
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Author:-
The alternative, there is a Regressor and an Isekai character.
Manga, ? vol, 2022
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Unless the Manga catches up, you will need to read the Web Novel.
Manga, ? vol, 2022
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Multiple Regressors
Manhwa, ? vol, 2023
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Author:8
The alternative, there is a Reincarnator and an Isekai character.
Manhwa, ? vol, 2021
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Author:-
The alternative, there is a Regressor and an Isekai character.
Manhwa, ? vol, 2023
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Author:-
The alternative, there is a Regressor and an Isekai character.
Manga, ? vol, 2024
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Author:10
Added as a special exception due to multiple POV tag.
Manga, 3 vol, 2011
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Manga adaption is based on the recommended Anime Original.