As with ships or cars, the convention is to refer to Jaegers as “she/her.” Kaiju are then called “he/him” or “it.” The distinction was originally made in the script so that I could use pronouns in the fight scenes without confusing who is who. Check out the difference below.
The kaiju lunges at the Jaeger. The Jaeger swats the kaiju away, sending the vast creature toppling through a cloverleaf interchange. The kaiju shakes off the concrete rubble and glares at the Jaeger.
The kaiju lunges at the Jaeger. She swats him away, sending the vast creature toppling through a cloverleaf interchange. He shakes off the concrete rubble and glares at her.
Regarding plural forms, I tend to say “kaiju” for both the singular and plural, as one would with “fish” or “deer” or “aircraft.” “Kaijus” sounds too awkward to me for some reason — though it is used at least once in the movie. I imagine that both forms are considered correct within the universe.