Kevin and his sourdough smell: given his unwashed nature (bacteria love sweat) and his numerous sweaty fat folds, my money is on intertrigo with yeast overgrowth/infection in the areas of intertrigo. Intertrigo is vastly more common if you have obesity (check), immune system suppression (check - the amhole is a Nurgle pit his body must fight every day), incontinence (check - he does that gross diaper thing iirc) and/or diabetes (I don't think we actually know this one way or the other, but his weight and diet are not conducive to good blood glucose levels).
Penny keeps bringing this smell up not because he lurrrves it, but in an attempt to shame Kevin into washing on the reg and treating the intertrigo. Unfortunately Kevin does not experience shame, so it doesn't work.
Intertrigo is actually pretty horrible and sore, and chronic and worsening and opens your body up to some real nasty infections in the broken skin. So if Kevin read this thread, which of course he absolutely does not, I would suggest washing his pits, torso and stomach folds, neck folds, underboob, and especially crotch and taint two or three times per weeks with your standard over-the-counter ketoconazole shampoo. Then dry very thoroughly and apply a thin layer of 50/50 standard vaginal thrush cream and an over the counter steroid skin cream to all the sore/red areas. Keep them as dry and sweat free as humanly possible. Cotton undergarments only and ideally only natural breathable fibres over and above that.