My novel Can Machines Bring Peace? has a wide range of characters. Every protagonist deserves companions, because no one can go at it alone. They bring surprises, emotional involvement, new solutions to problems.
You really can’t have a good adventure novel without them.
In my case there is Kira, the protagonist’s sister who’s been hiding in her room for months. Kuchi is an 87-year-old professor waiting to die. Aisake is a failed showman who’s constantly in debt. And then there is Mizuki, the daughter of the antagonist, Admiral Fujiwara.
Different ages, different backgrounds, different problems. They make the story crackle with electricity.