Celesca almost sees Hellscream as a younger brother. She taunts him relentlessly both in and out of battle, but doesn't actually think little of him at all. She'd really like to eat the souls that power him, but she can't override her coding even when she's starving.
Though she sees him as an equal to some extent, her defeat of him is something she likes to remind him of. Once she finds a way off of Celestella she plans to do it all over again and bring him back to Hell College.
A huge pain in the ass, but Hellscream doesn't really think all that poorly of her. She's an equal who can provide a challenge, and he can respect that. That's just more motivation to destroy her, anyway. Their battle banter has to be witnessed to be believed.
Both of them have a mutual understanding that they were created just to be pawns to their makers, but they feel too neutrally to really act on this knowledge and attempt to override their programming to do anything about it. For now.
Wow, she's pretty! And talks about eating souls a lot! She doesn't have any of those to put out, but she's always welcome to help herself to the snacks she leaves out for guests! They seem to invite themselves in a lot these days, but that's okay! She likes to make new friends!