Simply put, besides the Terran characters- which are very swiftly removed from the story from a protagonist's light- there is still plenty of characters to interact with. Firstly, Kerrigan herself is given a huge boost in character development via the prologue's events, going from wanting to forget her situation as the Queen of Blades to essentially being pissed off enough to go back to old habits, though she does seem to reduce the effectiveness of her ebilness (or at least, the barbarities that her broods have in mentality).
Then there's Abathur, your evolution master for the brood. Abathur has existed since the early days of the Swarm under the Overmind, then under Kerrigan, then back under our new tsundre Kerrigan. His mindset seems entirely locked on efficiency, and it's obvious that New Kerrigan is a bit surprised and probably upset at some of his actions to his own kind. Even still, while they argue a bit on some points, he's still needed by Kerrigan.
Kerrigan even has her own adjutant, a infested known as Izsha. She acts as Kerrigan's major advisory, although her robotic form of mind does seem to be a tad odd in comparison to the other characters of the setting. I'm not sure she's all she says she is, however, especially as she has many of old Kerrigan's memories stuffed in her.
