Tibby is perhaps the most remote of all of the many rather remote characters in Howards End – and intentionally so. As a consummate academic, he prefers to read about human behavior rather than experience it in the flesh, and all of his interactions reflect this perspective, even with his sisters (whom he loves, in his own special way – at a distance). Tibby is irritating, useless, pedantic, contrary, and dyspeptic (read: gassy), but all the same, we, like Margaret and Helen, simply can't stay angry at him; no matter how annoying he is, he's still kind of lovable in a weird way.
But why? Why shouldn't we just be able to dismiss Tibby as the most difficult member of a generally quite difficult set of siblings? The answer is simply that he doesn't exactly line up to our criteria of what makes a character (or person) good or bad in real life. Instead, his cartoonishly academic personality, likes, and dislikes make him into a bumbling, quirky outlet for comic relief. We are just as emotionally uninvolved with Tibby as he would be with us, and as a result, we're able to simply approach him as a kind of object of study.