42 kilogrammes? Is this girl even alive?
Well... Let us assume that she is rocking a BMI of 18.5 or so at her age*. In that case she would be 1.51 metres tall (just shy of 5 feet). While that's not very tall by western standards, it's nothing too rare for Japan.
The 44.2 kg at the beginning would have pushed her BMI to 19.5.
* talking from personal experience, I was running at 18.2 from late puberty until around 24/25, and I was nothing exceptional