Rebecca can mix herbs and chemicals, which Billy cannot. She also throws molotov cocktails faster.
Never leave the Hookshot too far behind. You will need it throughout the game, and forgetting it in an old area can lead to tedious backtracking. Combat & Enemy Tactics Resident Evil 0 HD Remaster
Conserving resources is critical, as enemies can be bullet sponges. Rebecca can mix herbs and chemicals, which Billy cannot