So, here's what happened this round.
Backstabs tries to attack the ninja!
Backstabs' roll: 6 + 8 (AGImod) = 14
Ninja's roll: 10 + 23 (Luckmod) = 33
Ninja dodges the attack like a pro!

Ninja chugs a health potion!
Heal roll (1d10): 9 (now has 53/70 HP)
Ninja heals 9 health.

Ninja attacks Backstabs!
Ninja's roll: 8 + 23 (AGImod) = 32
Backstabs' roll: 8 + 5 (Luckmod) = 13
The ninja hits!
Damage roll: 8 (1d20)
Ninja deals 26 - 3 = 23 damage!

Ninja holds back!
23 (damage dealt) - 14 (Backstabs' health before the attack) = 9
Ninja's roll: 5 + 23 (DEXmod) = 28
The ninja successfully holds back, knocking Backstabs out instead of killing her.
...
Robert waits.