Does a main character necessarily need to develop?

Basically, I've been writing my story for awhile and I've gotten the MC's character development, the quadragonist, and the tritagonist is a WIP.

As for the deuteragonist, however, I find it extremely difficult to really develop her. For the longest time, I thought it was bad that I don't have any development.

But for the story, it serves her purpose, as she's the go-to character for the other 3 protagonists while still maintaining a lively personality; she's there for the MC and the MC is there for her, she and the tritagonist get along well, and the quadragonist's development is really boosted because of her.

TL;DR - Is overt character development necessary for a main character if they already serve their purpose and still push the plot forwards?