Hi Beth,
1. 大写J looks like a scorpion in appearance (you can search the pictures online).
2. 大写J (特务J=Spy J) is very vicious and dangerous, just like the scorpion.
3. 大写J=the Singer(Jolin Tsai, from Taiwan, China), which might be kind of self-mockery; J stands for Jolin...
Hope my answer will be helpful...