Announced in the producer letter via the in-game announcement on the JP version, Final Fantasy VII remake will be having a collaboration with War of the Visions scheduled to arrive in April 21st. The new UR units confirmed are Cloud and Tifa.
From the announcement, it is mentioned that there will be 2 parts to the event, and Tifa is presumably the free unit. Unit details are still in development, so not much is shared just yet. More information will be revealed on April 17th.