What do you mean by that?
I thought they were fine. There was a lot of character building on
Spoiler
Lilliya and Isa Palant, it was interesting to see a Synthetic grow throughout the centuries, how she wished to be more human. Those bits with her and the Yautja further helped grow the character, showing how she regretted making certain choices and how she loathed being reminded that she was a robot.
Then Isa Palant, how she is obsessed with the Yautja, how her father told her things during her childhood which helped flesh out the character more.
Johnny Mains was also well fleshed out, how he and his squad are like a family, how one of them was inspired by the Yautja to join the Colonial Marines due to the aggressive combat oriented culture.
Then we have SnowDog, who has a drug addiction and how Gerald Marshal one of the Thirteen is kind of dangling it above her without making the blackmail too obvious.
Then that general of the Marines, who served in the Quaille Wars, who killed with his bare hands at one point.
Or how Gerald Marshal has a divorced wife and how he cares about his children, plus how he is humiliated by his weight etc, there's some subtle character development here and there.
This novel is far from having underdeveloped characters if you ask me, it just focuses more on the world rather than one character. It had a lot to explain since it's an entirely new era that's being explored, no other stories take place in the late 27th century, so there was lots to explore and flesh out.
I think book two will add more to the current characters while introducing new ones.
If anyone was underdeveloped, it'd be the Yautja to an extent as the author wasn't focusing too much on them to preserve the mystery. But from what we've got, it helped flesh out the race. The few Yautja characters all had their own personality, and were teased rather than developed, we were given scraps of development about the named Yautja.