Killer Instinct

You will unlock this upon winning 20 Ranked Matches with Mira. You can do this legitimately by searching for Ranked Matches and winning, boosting it with a friend by searching for a Ranked Match at the same time or 'double-boxing' with two Xbox Ones and a spare account. Keep in mind that you will occasionally get matched up with other players but it should be less likely now that the game's player pool has decreased significantly. You will unlock "Competitive Mira" after winning your first Ranked Match.

You can check how many Ranked wins you have under Fight Archives or Leaderboards.