Energy: the Ultimate gauge
Every character has an Energy meter that fills from a starting value up to a hard cap of 1,000. When the bar reaches 1,000, that character's Ultimate fires automatically. The bar carries between waves but resets at battle start.
Starting Energy varies by character
Most characters start a fight with 0 Energy, but some begin with a head-start:
A character starting with 600 Energy and 25 regen per second can fire their Ultimate within 8-10 seconds of battle start. A character starting at 0 with 20 regen per second takes 25+ seconds without other energy sources. This is one of the hidden advantages of some characters.