Co-Op Event 17: Sonic and Mario
From SmashWiki, the Super Smash Bros. wiki
Sonic and Mario (ソニックとマリオ, Sonic and Mario) is the 17th co-op event match out of a total of 21 in Super Smash Bros. Brawl. Player 1 controls Sonic while Player 2 controls Mario. The goal of this event is to defeat the three Marios and three Sonics on the Green Hill Zone stage. The foes will appear in alternating order, starting with Sonic.