Emoji is picture characters used in messages and webpages, such as ↑ ↓ ← → ★ ♀ ♂. Android 4.1 Jelly Bean includes some of the emoji. To enable Emoji and use it on Nexus 7, you can try this quick walkthrough.
How to Add Emoji keyboard to Nexus 7 Jelly Bean
* Go into Settings, tap on "Language & input" > Android keyboard.
* Tap on 3 stacked bars on the right and then "Add-on dictionaries" and select "Emoji for English words" to download and install.
* Now when you type words, the Android Keyboard will give you some auto-complete suggestion about Emoji. If you type "Star" and tap on the star character, this Emoji is inserted into the text.
There is limitation Emoji support on android OS. If you need more special "unicode" characters to display on the device, use the Unicode Map app to browse and copy them to the clipboard.