@Mijmij More like if you got me a strange weapon I can use and master it right away no problem, in other hand Saitama is if enemy exist I can kill it right away and instantly.
In this chapter rather than Manatsu turn into girl in body and heart in My opinion is more Manatsu merge with the girl soul of original owner body.
There are many stories that being possessed by ghost/spirit for long time is a bad thing or you can't having 2 souls with 1 body.