I know this is a long shot but here goes: I only remember one scene and can't tell if it was a LN, WN,film or a full blown series but there was a character (again not sure if it was the MC) he had some kind of ability to manipulate his biology (kinda similar to the MHA villain who can buff himself with muscles) and He wore a lab coat usually , he also had a pet dog he used to work with people but for some reason he ran away from them and then they hunted him down , when they closed on him he cut open his dog and somehow got inside and fused with it, when his pursuers arrived and wondered were he went their leader was patting the dog Anyway this is the only thing I recall appreciate any help
To your eternity - manga by author of silent voice- mc can transform into people who died after merging with them- but no above mentioned plot.