A/N: Not betaread, sorry, not got one for this fandom.
Written with their partnership, and sort of twisted friendship, in mind, though more could be read into it for anyone inclined to.
He watches her carefully at all times, seeing the ghost of a friend in her pale face at first. Everything about her is different but it is more a corruption of Snow than anything else; her movements made bolder and decidely deliberate, her voice raspier and letting more bite come out to play in her words, the cold emanating freely from her demanding attention lest she be dangerously underestimated. Caitlin Snow finally broke into too many pieces to put back together but Frost filled in what was missing, froze over the cracks, creating armor out of ice and standing up to the world. They have this in common at least, that for all their appearances they are not simply shadows of their former selves, because they learnt from their weaknesses and built themselves up stronger than ever.
He tells himself he looks to her because he wants to see what she is capable of (not because she is a piece of the home he was denied, stolen back from them). It's important he know what he faces should she betray him, that is certainly true, though the idea is barely a slither in his consciousness. Despite confidence in his abilities and his knowledge of the other timeline he still has to give it due consideration. Time remains fluid until that lynchpin moment that will break Barry Allen, leaving him able to remake himself in the darkness it spawns in his heart. He can't risk her reverting to form, back into a friend of Barry, so she must be twisted into his ally instead. He must show her an alternate way forward by accepting her. It isn't hard for him, he knows very well what power acceptance, and the bitter converse, holds over people like them who once felt too keenly its grasp.
He wonders if Barry knows what she can do; he doubts the preoccupied Barry has paid much attention in the months he still had Caitlin Snow to the details Savitar is taking his time to catalogue and analyze thesedays. Not that Barry had much opportunity to. Snow ran from her powers, hid it all away from her friends in fear of what she was becoming. Killer Frost is the equal and opposite reaction to that, relishing her power, showcasing what she can do with delight and inviting intimidation to surround her.
Savitar can recognise curiosity plays a small part in his desire to observe Frost but the purpose is larger than that, she is another weapon in his arsenal and he wants to know what will happen if he chooses to pull that trigger. He wants to know exactly how she can make them suffer, even as he already knows that the worst suffering she can inflict is infact emotional, already wrought by her continued absence. Team Flash's loss of Caitlin Snow is a wound in itself, one opened a little wider, the agony renewed, each time she fails to come home to them. He knows how sharply it stabs at Barry because he feels the echo of that pain in their shared memories. Each time she will choose him and it is a delicious victory that he savours against the burn of their future rejection. Out of all that he has lost, there is satisfaction he has taken from them in return and will again when he kills Iris. Any ache that persists in him from being the cause is a drop in the ocean against everything he has been put through. They deserve it (and he ignores how that must mean he believes he deserves it too, how his hurt means he cannot see clearly a way to break out of this vicious cycle).
Frost doesn't look to him with fondness, that much he expected. Her strength comes from detachment much like his own. They can't risk caring any longer because caring is what broke them in the first place. The closest to feeling he ever gets is anger and hate rushing in his veins, that he lets creep in purely because it is the fuel to his plan, something to keep him going when the need for patience wearies him. He brandishes that fire in the same manner she casts her ice, a barrier between themselves and anything that could throw them off course. He can't risk forgetting what they have done, what he is yet to do.
He doesn't look to Frost with fondness, though familiarity lingers there, because she is not his friend and he is not hers, even when they have found out she retains more of Snow's compassion than expected. He is grateful she does not call him Barry except for the one time, when he takes the opportunity to cut that idea down and make it very clear neither of them are the people they might trivially resemble.
Those memories that bind them close are the real ghosts, something to fear far more for what recall they bring to mind of times past and something that won't ever be again for either of them. They are not each others saviours like they once were. Yet the past can't be wiped clean, their messy existence proves that and these people they are now, forged by the future, are what will drag them out of the hell of Barry Allen's creation. It's the one promise he makes once he is free of his prison and it is not made to Cait - he waits until the time is right, for Frost to be, so that she alone can accept his terms (like he knows she will, though if he looked closer he might see he depends upon it just as much, on her accepting him).