Regen is applied at end of turn, similar to Poison.
Chance to buff ATK/AGI if above 50% HP is 100% (SL1-7)
ATK & AGI Buff are static at +1 (SL1-7)
The way Topaz's skill works, the Regen "Shield" is applied, and at the end of the turn, you're healed for X% HP, and if your HP% is above 50%, then the Party gets buffed +1 ATK/AGI.
Ideally, if you're able to keep the Regen effect for all 3 turns (and keep above 50% HP), then it equals out to +3 ATK/AGI for the duration.
Funny enough, these effects do stack if used with an AoE Enhance card (so everyone would Regen health at the end of the turn and grant buffs accordingly if they're above 50% HP).