Odette had 100% MR on her event (as an enemy)

There is also a boss golen with 60K HP, 3500 DEF, 60 MR, 5200 ATK

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Nevermind, Odette had 100 def, I looked at the wrong wiki square

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But the map itself made everyone with 90% MR

"Your team's magic damage becomes progressively weaker in the event maps, as follows:

Monster Extermination E: 0.7x

Monster Extermination N: 0.5x

Monster Extermination H: 0.3x

Monster Extermination X: 0.1x"

http://millenniumwaraigis.wikia.com/wiki/Mystic_Sorcery

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It is possible to get Sophie up 80 MR on DMM. This requires having another unit with the awaken ability of giving all units +10 MR. Additionally, Sophie's awakened ability will then double her MR when active, causing it to go up to 80 (normally it would be 60 due to her 30 MR).

As for the highest natural MR among obtainable units though, that would so far be Cynthia with 40 MR on her own (which can be raised to 50 with the previously noted unit).

There is also Odette who has a skill that at max is akin to giving units 20 MR, but it actually does so via reducing the attack of magic enemies by 20%, thus if used with a Sophie possessing 80 MR would presumably result in her taking 16% (20% of the reduced 80%) of the total damage rather than 0% (admittedly as I don't play on DMM I have not tested this to confirm).