Some classes have more dramatic, obvious powers than others; some are front-loaded or back-loaded. The Heir is a powerful, frontloaded class, which makes it ideal for a class like the Seer--no obvious showy powers upfront at all--the deploy to achieve things. Both are passive classes, but passive in different ways; they cooperate to help each other as teammates.
It also makes for a different, but equally effective, pairing from the Seer-Knight we've seen a lot of. Seers and Knights both seem able to function independently, but work better in tandem. Seers are strategic, while Knights are tactical; they can act on their own with ingenuity and planning, but seem to lack a bigger picture. Heirs are more like special forces to be deployed. The only time we ever see Equius not following Aradia around like a puppydog, he's on Nepeta's world, completely at loose ends. The Heirs NEED their friends.
One interesting thing is that classes that are both powerful AND active tend to be the "troublemaker" classes, it seems like. Princes, Thieves and Witches seem prone to create as many difficulties for the team as they solve, especially since they are reluctant team players in the first place. Although the same seems to be true for Bards--but then, because their powers are destructive, using them for/on others isn't necessarily a good thing. And as wild cards, while they aren't self-directed, they aren't really a part of any plans, either. It's an interesting solution to the problem of balance.