Those saying everyone hates Umi: In some cases, probably true.
But in most cases, I think "distrust" would be more accurate. You can't tell me that if Umi's soulmate Mami suddenly started acting unexpectedly nice, you wouldn't be deeply suspicious of her motives, and it's just as valid for Umi. It's not a given that he plans anything - but for now they are still at a party he organized for Chizuru, with lots of his friends there. They are still in the tiger's den.
Kazuya has a very bad habit of closing his eyes to avoid having to stand up for himself. It would be just as much a mistake for him to decide all's well now, as it was for him to decide there's no reason to be suspicious of Mami suddenly hooking up with Grandma.