Ellysaw is an assault frigate. They are equipped with two light plasma cannons with decent range. They reside in the middle-ground between corvettes and destroyers. Their main use is defending against destroyers, as their range and speed allow them to outmaneuver the destroyers, though you need to use them in big numbers to pull that off.
Cost-wise, you can make two Ellysaws for every destroyer, so it is a good investment for fighting against them. However, they get easily destroyed by submarines, against which they have no weapon.
Ellysaws have a lower cost, but weaker armor and weapons than their CORTEX counterpart (Riptide).