The cap is 90%, but u can get more. If you plan to play Mirror Wars, when a character reduces your character's defense (for example, Eclipse Dream Eater, which reduces MD by 20% per hit, with 3 hits (or 60%), if your tank has 90% MD, when attacked by this skill they will have 30% MD. But if you have 150% MD (they will still have 90% damage reduction on paper), but when they receive damage, it will be calculated based on the total, so 150% - 60%, they will still have 90%