Nexos are on the highest level of alert.
They are actually antagonistic toward the other alert levels.
Nexo Warriors are the basic grunts. They are equipped with a Disintegrator Ray-like weapon and jetpacks. Yes, jetpacks. Crypto is no longer safe hiding out on rooftops. Warriors are on-foot only.
Nexo Walkers are the three-story tall monsters of doom. They have a similar weapon and a powerful laser. If they aren't engaging Crypto, they will actually wander around and shoot cars and people on their own. It's funny to watch. Walkers appear both on-foot and in-air.
Nexo Saucers are flying enemies that shoot homing Plasma Cannon shots at you and typically fly in some kind of formation. They used to fire a devastating Quantum Deconstructor as well, but that was cut because it was no fun. Trust me, it wasn't. They only appear in-air.
However, Nexos are not always tied to the alert level. They can definitely appear on their own in the course of a mission, so always be on the lookout for them!
The final Invasion Site has its very own alert level consisting of color-coded Nexo forces.
Actually you meet the smaller, easier blue nexos first which we deemed "Nexo Teens" around the office. And then higher alert gets into a mix of blue and the green Nexos you see on Earth sites. Then the final alert levels have the big red Nexos. It ramps up appropriately.