A Marvel Rivals team that works is not six strong heroes thrown together. It is a lineup where the roles cover each other, the picks counter what the enemy is running, and at least one team-up is actually online. Role balance gets you a functional team; the team-up layer and the matchup layer are what win the fight.
Start from role balance
Marvel Rivals has three roles: Vanguards up front, Duelists for damage, Strategists keeping everyone alive. A balanced spread, commonly two of each, is the baseline most comps start from. Skew too far and the cracks show fast. Too few Strategists and your team melts in extended fights; too few Vanguards and there is no one to make space or soak the enemy dive. You can flex off the even split for a reason, but do it on purpose, not by accident.
Build around a team-up
This is the part Overwatch does not have. Each season, Marvel Rivals defines a set of team-ups, pairs and trios that unlock a bonus when the right heroes share a lineup. A team-up turns two good picks into a combination stronger than the sum, so the best comps tend to pick an anchor team-up first and fill the rest of the slots around it. Decide which team-up you want online, lock the heroes it needs, then choose the remaining picks to cover the roles and matchups you are missing. If you are not sure which pairs are pulling their weight right now, the synergy data on each hero page and the best duos tool both rank pairings by how they actually perform together.
Cover the matchup and the map
A team-up anchor still has to survive what the enemy picks. Once your core is set, look at the counters. If one of your heroes is walking into a hard matchup, a small swap that keeps your team-up intact is usually worth more than forcing the original pick. Maps matter too, since a comp built around zone control loses value on a layout full of flanks, and a dive-heavy lineup struggles in tight corridors. The goal is a comp that holds up on three axes at once: roles covered, team-up live, matchups and map in your favour.
Let the tool do the math
Checking all of that by feel is hard, which is what the team builder is for. Drop your lineup and the enemy lineup in and it scores the comp on how it counters the enemy (the heaviest factor), how its heroes synergise, and how it does on the map, returning a single win-chance read plus which picks are helping or hurting. Use it before you queue to settle a stack's composition, and once the match starts the Counterwatch app overlay runs the same scoring live as picks lock.