You might need to complete certain challenging quests or reach specific levels to unlock unlimited characters. But it depends on the game's specific rules and updates.
Marcus Fenix is extremely important in both Gears of War 1 and 2. He's a skilled and tough soldier. Dom Santiago is also crucial, especially in Gears of War 2 as his personal story about finding his wife adds an emotional layer. General RAAM is a significant Locust character in Gears of War 1, being a powerful and menacing foe.
Kait Diaz is also a main character. She has her own story arc and contributes a great deal to the overall plot. And Del Walker is another important character. He is often by JD's side and they face challenges together. They all have different personalities and skills that are explored throughout the novel.