The only way Hiro can raise his Character Level is by improving his relationship with his teammates.
On top of having to manage his relationships with all of his companions, Hiro also has to consider his relationship with important NPCs like PrincessVelvet, her sister Satine, their maids Minako and Kaya, the head of the knights Sir Isaac, his best man Kelvin... the list goes on and on. Some of them can even join the party if he plays his cards right...
Hiro's friends might decide to leave if their approval isn't high enough.
Hiro is a very specific kind of Guile Hero: On his own, he doesn't have any special power nor proficiency in any physical nor intellectual skill... However, he has a great degree of charisma and empathy, so he uses these talents to recruit adventurers capable of helping him in his journey,
Hiro is a lone wolf and derives his power from that. As his teammates gain more trust in him, he gets less powerful.
Gaining levels and abilities helps Hiro become closer to his allies.
Subverted: While Hiro can pursue relationships with his teammates, none of these choices have any impact on actual gameplay.
Double Subverted: While it appears at first to have no point, this actually effects which of the ending paths are open to Hiro.
Averted: Hiro has no teammates or friends, so there are no such relationships.
Deconstructed: Hiro comes to view his friendships as merely means to an end; as soon as he judges he's gained all the 'rewards' for spending time with someone, he ditches them to focus on others.
Reconstructed: ...But is forced to backtrack when those 'rewards' are suddenly taken away from him. Hiro has to maintain his relationships if he wants to continue to enjoy the spoils.
Played For Drama: Some of the relationships Hiro can have are mutually exclusive, or practically so — for instance, the Lawful Good Kelvin wants to arrest Mark for his myriad crimes. Hiro must tread carefully if he wishes to befriend both, or risk losing one or both.