Belos probably cares for Hunter in a "pet raised for slaughter" kinda fashion.
Now, this isn't an exact analogy; the Emperor hasn't raised his nephew under inhumane conditions nor fully dehumanized him, even though the boy is an asset to him. A possible sacrifice too, seeing as Hunter is going to play a significant part in the "Titan's plans." And the Emperor won't hesitate; we are talking about a man who uses people however he sees fit in order to bring his ultimate goal into fruition.
Even I was never under the illusion that Hunter would ever take precedence over Belos' plans when I still thought he cared about his nephew as a person. Yet, Belos doesn't mind having Hunter around when he and the boy are discussing subjects relevant to Belos' interests – not Hunter's interests – or during his moments of vulnerability.
As if the kid is some kind of "trusted pet" who's there to serve his needs and commands, but not be heard.
Hunter's emotional needs? They matter less than his. Basically, this:
"It's nice having you around, Hunter. You're a trustworthy person." *Pats the kid's head like he's a dog.* "But please, can you not become an inconvenience? It's such a hassle, listening to your worries whilst dealing with my own. I hope you understand."
I know some people have compared Belos to Ghetsis, and the parallels are there, but at least Belos is genuinely friendly with his "pet." Ghetsis' Hydreigon hated him.
I don't think the Emperor's gonna turn out to be a complete monster.
I know what you're saying and I agree.
"Belos is an abuser" and "Belos is somewhat fond of Hunter" are both contradictory statements that can co-exist.
An abuser doesn't have to be a complete monster devoid of feeling just someone who doesn't love you enough to put your needs above their own. To the point where you would even hesitate to call it love.
Op isn't saying that Belos' kind demeanour towards Hunter isn't being used as a manipulative tool (it is).... just that there is a possibility, to some degree, that Belos' charade of reminiscing about the human realm was genuine.
Implying he can speak openly with Hunter because he may view him as more than a mere pawn, but never quite a son, etc. Hence the analogy of their relationship being pet-like.
For example, Belos saying "Goodnight, Hunter" was a clear dismissal but his tone implied he was amused and found fondness in the boys need to prove himself.
He actually takes on a similar tone with Luz in "Youngblood, Old Souls" after she manages to break off part of his mask.
So to reiterate your thoughts for everyone else:
Belos is an abuser manipulating Hunter for his own gain but appears somewhat genuinely fond of him like a pet you know you're gonna outlive.