Can we ever have a deeper interpretation of the relationship between Negative! Penny and Negative! Nigel?
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First of all, it's PENELOPE. Never call her any stupid nicknames.
Long story short, Penelope and Nigel can't stand each other, but we need to go a bit back to understand the whole situation.
Penelope wanted to get the DNK back and rule the world. Do do that, she needed to win Wally's trust. And to do that, she used Joey and, after finding out Wally himself seemed interested in him, she used Nigel.
Wally was actually impressed by her plans and her determination, so he gladly allowed her in his graces, and she helped him get back into the treehouse and slowly re-establish his regime.
Nigel didn't take it too well. When Penelope's around, Wally didn't allow anyone to even touch her. She came before anyone else. Even Nigel. And Nigel couldn't stand that, because he wanted Wally to have eyes for him only.
Penelope knew it too well, and loved to taunt him with the fact she's closer to Wally than he'd have ever been.
Nigel once snapped and hit her.
He will never do that ever again after Wally got really mad at him.
In the time before the whole Adult AU meeting, Nigel was forced to take care of Penelope, following Wally's orders. Note that Penelope can actually age up here.
It was bad for both of them. Lots of screaming and fighting.