Who are your other options? Do you have 5* the champion? Do you have and use Morningstar? If so, he's worth rank 2 in my view if you have no other mystic 6* options.
Particularly as t5b is so much easier to come by, he's duped and he's got good synergies.
It really depends on the rest of your roster, but personally I would. He has 2 immunities Coldsnap and Poison, he also takes 0 damage from all incinerate effects, and instead gets 12% of a bar per second, and heals off it. He has mediocre damage but this can be fixed from synergies, he can tank sp3 and has a regen which saves him, he can prevent buffs, and is the best for the 1% regeneration node. Use his sp3 to power drain 2 full bars of power in which he gains a permanent aura which is great for power management and damage. Overall he has quite a bit of utility and is worth the rank up if u need him for a certain thing.
I definitely think Mephisto is worthy of R2 - everyone loves Falcon now but Mephisto has been OG Fate Sealing for years now and has regen, incinerate damage and poison/coldsnap/incinerate immunity.
Having said that, I would save the gem for another champ, especially because you already have max/overflow of Mystic T4CC
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Particularly as t5b is so much easier to come by, he's duped and he's got good synergies.
Having said that, I would save the gem for another champ, especially because you already have max/overflow of Mystic T4CC
Only if you actually like him though. His damage is meh but he has good immunities and a safeguard.