An example of a good Match up is Andromeda, The Archer, and the item Stone Heel totem. Andromeda has only far ranged attacks, with the ability to reduce her movement cost by Five. Stone Heel totem Increases the cost of motion for enemies by five. This combo does the double duty of shoring up Andromedas weakness by making it harder for you opponent to move, and enhances a strength by allowing her to keep the enemy at bay.
An example of a bad pairing is trying to match up Tafari, The Trapper, with Mindreader's Chalice. Tafari shoves your opponent into a corner by preventing your opponent from switching. Mindreader's Chalice Boosts the damage of intercepts and gives you back 30 energy when you intercept succesfully. This item is normally a solid (If juvinile) choice for an item, but Tafari renders it useless since your opponent can't switch out anyways.
RECAP: A good item for a character Shores up a weakness, or enhances a strength. If the item does neither for a character, no matter how cool the item is, PUT IT ON SOMEONE ELSE OR DON'T USE IT AT ALL!!!!!!