I was unable to reach Rank A (I tried It on Chihuaua and then on Driller's routes), even doing all the choices as it says. Guess I'm doing something wrong, but I'm unable to find out what it is.
To get an A rank, you should have everyone helping you out. Including Syd.
To get Syd to help in the ending, you should let him use the generator and "Take the oportunity and take a look at what’s hidden under the pink fabric" in each route.
To get Driller to help, you should be agree to a truce and have roughly >90 LovePoints.
Chihuahua will always help, but if don't build his confidence or love he will die.
Lascivion/Jake will escape if you choose the wrong choices, such as choosing Attack rather than Negotiate.