Or perhaps it was his body before he got isekai-ed. This going to get real dark real fast but the fantasy world of an isekai is now generally considered to be all in the head of person who was hit by truck-kun as they hover in between life and death. A sort of purgatory if you will. A good life action vision of this trope is the TV series Life on Mars and its follow up Ashes to Ashes, they well worth a watch if you can find them.Oh....oh shit....
What if this was all planned from the same beginning? What if Takeru didn't inhabit the boy's body, but was perhaps some kind of magic clone that woke up to the Hero's ego when he attempted suicide?!
Honestly, that'd be outta left field. Cause I really want Edleweiss to be evil. If she isn't, then reincarnation as you mentioned.hmm. the 5 of them are 'predetermined reincarnations' so I assume in the future we're going to get flashbacks to people who left an impression on the hero.
Which can connect to the 'prophecy'
so it was a trap...