Marvel Rivals has three primary roles and every playable character falls under one of them.
Duelists are primary damage dealers. These characters excel in dealing high damage to enemies, even if they may be fairly squishy
Strategists are Marvel Rivals’ version of healers. These characters work toward ensuring their teammates last longer in fights, whether through healing, shielding, or even complete resurrection
Finally, Vanguards are tank characters. As the name suggests, these characters stay at the forefront and take the brunt of the damage dealt by enemies, though they also usually have the health to back their role up.
Absolutely! While a team of all Duelists and Strategists could theoretically win, it would be extremely difficult.
Vanguards sponge much of the damage intended for other players to ensure their teammates can finish the job, and not having at least one tank means there's nothing stopping enemy Duelists from absolutely destroying the entire enemy team using their ultimates.