Like for example being transmigrated to a fantasy story where there the bestfriend/side-kick/comic-relief character of the main character?
Hell yeah! "Is it though to be a friend" is what you're looking for, 2slow2late is translating it and it's pretty funny!
I feel like this might fit your criteria. Maybe. https://www.novelupdates.com/series/i-was-a-sword-when-i-reincarnated/
Mother of Learning - MC gets dragged into a time loop after he gets hit with soul magic with Zach which slightly bonded their soul Spoiler It is technically an isekai Tensei Shitara Ken Deshita - MC is literally a sword
Yuusha Yori Saikyouna Kuro Kishi - MC isnt the hero but the hero(her harem protag friend) isnt summoned yet.
Rather than side characters isnt the first one the rival character while the other is a minor villain of the original story?
No ntr, it's just that the females change their male target... it's just logical if you read half first novel and remember "who is the real mc"
In Amber Sword, MC was reincarnated into an NPC that was supposed to die in the game world. I don't remember him being a side-kick though, so I'm not sure if it fits your criteria.