These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: Diana is a remorseless commander of the Visitor invasion forces and by far the worst one seen. During the first occupation she planned the entire operation and was the real powerholder in the fleet. She engineered the genocide of the human race and invented both the process of harvesting humans for food and the mind-raping "conversion procedure". She oversees these tortures herself simply because she enjoys them, during which the victim is haunted with vivid nightmares until they've either been brainwashed into obedience or rendered a vegetable. She performs lethal experiments on captured prisoners, kills her own superiors to take their place and orders Brian to implant Robin with his alien fetus as an experiment, leaving her a traumatized wreck. As the Visitor fleet is leaving, she goes ahead with trying to irradiate the Earth with an atomic bomb even after they've already lost and despite John's disgust with her pettiness. After being captured for war crimes, she escapes back to the fleet and launches a second, much more destructive invasion of Earth.
Moral Event Horizon: Daniel Bernstein steadily becomes worse as he joins the visitors, using his newfound power for petty acts of cruelty and to threaten other people to get what he wants. He eventually kills an old woman who was spying for the resistance in the Visitor Headquarters, despite her pleas to him to let her go and how she's known him since he was a kid. The moment is portrayed as the point of no return for him, and the resistance sets him up to be killed when they raid his house later on.
The Woobie: Willie was a minor part of the original miniseries and The Final Battle, but was so popular with fans they made him a regular on the subsequent television series. He's one of the first of the Visitors to join the Resistance, he speaks broken English ("English is not well to me. I learned Arabic for going there.") and at the end of Final Battle his human girlfriend dies in his arms after she takes a laser gun blast in the chest to save him. He's got a perpetual hangdog look and in the series he wears a bow tie!