If possible like winning a ticket, you should ask 
Yuu Watase to draw you a character plus her 
signature.  If she is willing to do so, draw it on 
a sign-board or a nice quality of paper instead.

Even for a signature, it's not good to sign on a 
cel.  Even for the permanent ink pen, it does stay 
on the cel surface for a long time, but if you put 
a plastic bag storing with the cel...after many 
years, the ink will eventually stick to the 
plastic bag together, and you will be screwed 
later.  It happened to one of my cels before.