From February 11 2016 Grand Master Yoda has become available has an unlockable character in a seven day time limited event.
EDIT: This event is being run again in March 2016!
He is not at all easy to unlock as you will need to collect 150 Yoda shards, these are only unlockable if you have five Jedis. You can then compete in training battles against Yoda.
Each battle gains you 25 Yoda Shards, but to get access to subsequent battles, the levels of your Jedi's must have a sufficient star level.
The game has stated that although this is a limited time event, there will be other chances to unlock Grand Master Yoda in the near future.
Here is a screenshot of Yoda with a look at some of the abilities he will come with if you are lucky enough to unlock him:
Here is what he looks like in action against my team of Jedis.
Thank you for telling me abaot unlocking yoga now I can tell my kids