Emoji - to paraphrase Mr S Jobs - is a bag of hurt.
The three carriers (docomo, AU and Softbank) have their own standards all using different pictures.. so one smiley face on one carrier looks different to the other 2.
For many years the system were completely incompatible with other carriers. ie If I sent an emoji from docomo to AU the picture would not be rendered. Similarly if I sent an emoji to a desktop computer it would not be rendered (without downloading the appropriate plugins - which to my knowledge are windows only).
Around 2003-4 the carriers all go together and agreed to keep their systems.. but to translate and render with the appropriate version on the receiving phone - ie docomo sender sends docomo smile

to his AU and Softbank buddy they would see

and

respectfully.
So inter carrier compatibility was somewhat established. There are still several emoji on all carriers which wont translate across.
gmail has recently enabled it and so using gmail (via the web) you can send gmail versions and receive the original carrier version as sent from the phone.
hope that helps a bit.