I've only started watching the fifth season.
I think the moment that cemented, "What is he at?" for Trent for me was the dialogue he and Daria had at the end of the episode where he failed on helping her and Jane with a school project. The conversation seems obviously meant to be the writers going, "This relationship would not work." but then after it Trent kisses Daria on the cheek. Although she doesn't blush or anything so I assume it's meant to show her feelings for him being gone.
The two do seem close in friendship and supportive of each other. The highschool thing could turn him off or maybe he just knows what Daria is like and a relationship with her would be more complicated that what he wants. I can't really see him being as sensitive to her ethics and beliefs as Tom is without pulling a, "Fix The Guy" type thing.
I'll stop there. I think I could write an essay on this stuff just about Daria and Trent off the top of my head.