Deimos likes how determined Amber is. Her honesty about what she likes and wants. Appearance wise, he likes the BIG red Oni lady. In a way, he sees himself in her, that selfless desire to help people. She's a woman that knows what she wants, fight big bads and monsters, explore the world and see new discoveries.
He likes her dumb laugh, the way she smiles, her horns. Just seeing her makes him happy.

As for Amber, there's a certain intrigue to Deimos that drew her in initially. He was different in a way that made her want to partner up with him and learn more about him.
That borderline reckless desire to help people, that unwavering will against threats. And then he ends up acting like an awkward goof at random times. To Amber, Deimos is an enigma that she wants to learn more about.
Certainly helps that he's easy on the eyes, fit in a way that ticks a lot of Ambers boxes.
Of course when she realizes she likes him more than that, she's embarrassed. Granted they both are, but they keep hanging out together until one of them says something.

Of course besides being attracted, they'll go through too much together to ever consider leaving the other. Promising to help each other with their problems and getting them through tough times.
In a way they're meant to be with each other.