Aloy's purpose is to deal a huge amount of damage through her Burst and the Melt reaction.
Mona buffs Aloy's and the team's damage with her Burst's Omen effect.
Sucrose gives free Elemental Mastery to the team and enhances reaction damage, as well as shredding elements with 4pc Viridescent Venerer.
Bennett's utility consists in providing a huge ATK boost to the team. He can't really be replaced as his contribute is unique.