Tyr is pretty essential for Thunder right now, but Thor will eventually overtake her. An argument could be made for Awakening either. Certainly Tyr's Awakening will provide more immediate benefit, since it upgrades her debuff from terrible to decent.

Also, I never really understood why people judged SSRs by base stats, especially on the DMM wiki. Adjustments from weapons and eidolons completely dwarf the differences between characters, especially of the same rarity. Although, having especially low HP does make a character the first to die from multiple AoEs, which is generally undesirable. But that's fixable with accessories.