PRESS THE FLAG TWICE. TWIIIIIIICE Player 1, or "It" is Pikachu. Pikachu uses the cursor to move. Player 2 is Eevee. Eevee uses Up/Down/Left/Right or WASD to move. The NPC player is Shiny Eevee. Shiny Eevee uses its own code to move, but the movement can be influenced by Pikachu.
Basically, if you have never played Tag, "It" which is Pikachu or Player 1, must "tag" everyone, or touch them, to win.