The fusions available in-game have not been listed as of yet, and for now, we only know that Stevonnie will be playable. The fusions will tie into the game’s recently revealed relationship mechanic, which involves a meter that builds between two characters the more they work together in battle, mirroring the themes of trust and emotional connection associated with fusion in the show.
When the relationship meter between two characters fills up, certain characters, like Steven and Connie, will have the ability to briefly fuse in order to take different approaches in battle.
Steven Universe: Save the Light is planned for release this summer and is coming to PS4 and Xbox One, with other possible platforms being revealed in the future.