SeaFlames

The glistening water of the grand seas covers this extraterrestrial visitor as green flames magically burn in the water. It is unknown to all, as the mystery of it's flames not burning it hide in the dark.
The flames are its signs of life and as long as they burn it shall thrive. It's watery skin lets it slip through small spaces (this does not actually happen in game) and keeps him hydrated (this does happen in game).
The fire and water together create powerful energy so it can move and can catch it's prey with the blink of an eye.

As your soul fuses with this creature, sword in hand, you set out for a big adventure; exploring the mysterious kingdom of Minecraft.