I imagine in that particular set-up Leona in part yearns not only for the individual people but for the clear love and tenderness their relationship holds, to be part of the dynamic
Which could probably come with a type of weird jealousy as he also wishes to occupy the spaces they hold, this held simultaneously along with his feelings for both Trey and Idia could lead him to resent himself for feeling like he's selfish and unable to let those he loves find happiness with who he knows, on account of his own affection, is a good person to be with thus positioning himself as a destructive force haunting them individually as well their togetherness in his mind
At first, I feel like Leona would have these strong feelings for both Trey and Idia. He denies whatever he's feeling inside, instead he opts to bottle his feelings and push them away.
Leona hates himself for feeling this way and feels like a bother to the two who are already in a happy relationship. To the two who clearly don't need him.