The letter was short but precise, and I immediately understood it. Janus has been captured before we can say anything to him. And based on the words on why he readily agreed, Nobuhiko must have used one of the Cure All medicine they have developed and used it on Janus's sister, curing her problem in the process and causing Janus to own a favor for Nobuhiko which the latter immediately used for his advantage.
And since his sister is writing this letter, I can also tell that they are either under surveillance by the security of Nobuhiko and he is held somewhere where he will not be able to ask for any kind of help. Basically, a forced labor.
As to why he choose Janus, it's because Janus has been with me in many battles and they are using me as a basis to get more covered ground in finding the Tree of Life. They are gathering those who are skilled enough to find it.
