Rank

Ranks are letter grades that are applied to each of your units' stats (Damage, Cooldown, Range). 
You can reroll all of the ranks on a unit with Risky Dice at Rank Rerolls next to Summon. If you want to roll your ranks again, but without rolling your preferred rank, you can use Frost Binds. Ranks are given automatically when Unit Summoning.
Underneath is a table showing all the buffs/nerfs each rank deals to a unit's damage, cooldown and range. 
*-10% Cooldown means LESS cooldown (shorter period of time between attacks). Wheras +3.4% Cooldown means HIGHER cooldown (higher period of time between attacks)*
Rank	Damage (%)	Cooldown (%)	Range (%)
S 	+20% 	-10% 	+10%
A+ 	+19.6% 	-9.75% 	+9.75%
A 	+18.8% 	-9.4% 	+9.4%
A- 	+18.1% 	-9% 	+9%
B+ 	+17.4% 	-8.6% 	+8.6%
B 	+15.9% 	-7.9% 	+7.9%
B- 	+13.6% 	-6.8% 	+6.8%
C+ 	+11.2% 	-5.6% 	+5.6%
C 	+8.2% 	-4% 	+4%
C- 	+4.7% 	-2.5% 	+2.5%
D+ 	+1.5% 	-0.8% 	+0.8%
D 	-1.5% 	+1% 	-1%
E 	-4.8% 	+3.4% 	-3.4%
F 	-8.3% 	+7.4% 	-7.4%
== Worthiness ==
Higher Worthiness = better chance of higher Ranks. 1% worthiness = 100 unit kills, 100% worthiness = 10,000 unit kills. Worthiness resets on roll.