MultiVersus update 1.03 is out today with the launch of season 1. You can get the complete patch notes here.
This patch is bringing Morty as a new character as well as other surprises. The free characters will also get a rotation beginning August 15 until 30. The characters that are confirmed to be rotated include Arya, Batman, LeBron, and Steven. There are plenty of other nerfs to the characters as well details about which can be seen below.