Lotsa things are going on here. The kimchi (with no work on your part, unless you're fermenting your own kimchi... I'm HELL Not) adds a spicy, tangy complexity, cutting the monotony of an otherwise just plain rich and decadent grilled cheese. The extra layer of savoury and toasty cheese crisps enrobing your toast adds yet another textural contrast. More contrasting notes, More fun - just like in humans.
2 slices of bread
2 Tablespoons Mayo
Some Kimchi, chopped up
Cheddar Cheese (or any melty cheese of your choice), shredded