UPDATE 4/10/21: Smash Bros. boss Masahiro Sakurai will reveal the final character for Super Smash Bros. Ultimate tomorrow at 3pm UK time, in a presentation lasting 40 minutes.
Nintendo has also said we'll find out exactly when this last character will arrive, and invited fans to speculate on who it may be. The consensus seems to be expecting Sora from Kingdom Hearts. But we're still rooting for Birdo.
