if you have a mac, plug in your phone into itunes and rename the folder through itunes. the command for the symbol is press and hold option+shift and press the 'k' key. you can also open up the character viewer to choose from a bunch of other symbols.
if you don't have a mac and or you want to use the apple symbol (among other things), launch safari in your iphone and go to http://mrgan.com/gb/
the glyphboard allows you to copy and paste the symbols.
On your iPhone you can just copy and paste from here.