Nintendo UK just posted a ARMS trailer revealed that we saw from GameXplain's discussion that this green thing was in the previous Direct along with the other 3 unknown fighters in ARMS. This green fighter is called Helix and he is actually a Slime Fighter. (Unfortunately, I did saw the ARMS thread was in this section, but the last post was in February 2nd and we can't post it the thread since it's more than 30 days old.)