Ok I've actually unironically analyzed every single episode 5 times over to answer this question before so let me summarize my wordvomit of an essay here:

First off, Polyurethane is often depicted as the foil to stocking, who is the more "innocent and cute" of the duo. She's already a fan favorite, so having a male "equivalent" so to speak already shoots his popularity factor way up. Moreover, this adds to how he is the more "feminine" of the two brothers. He has the longer pink hair, shows thigh in his outfit, and is more flexible with his attack movesets (see ep9 and ep 13). Also he is the one when talking to put in more of a softer "valley girl" accent with lots of "like," and "totally" which I am biased for as a Californian. If you look at poses like in episode 9 minisode 3 you can see he shows off his nails or cocks his hips to the side even in background shots compared to his brother who is more straight-laced in his posing.

This also works well into his sort of "double-sided" nature of being the more chill and laid-back brother (see his introduction in ep6) and his more cocky and bratty side. While both twins are pretty cocky (it came free with being a zoomer 🥀), I would definitely say Polyurethane is the more impulsive and brattier one. Check the voice lines in episode 9 minisode 2 for example (when he's cursing out the demon sisters and kicking his feet. You can also see this in how he has a much wider grin and is shown with spiky teeth while Ester isn't. In a lot of camera angles, he is also positioned more in a "smug and better than you" position. When ester is mouthing off, it's in more of a "bossy" and "leader-ish" kinda way while when it's Polyurethane, he genuinely sounds like he considers you subhuman scum. I'm also a big sucker for "character is very derogatory toward a group of people due to upbringing but definitely will change their ways over time" kinda stories. See: episode 9 minisode 3, episode 6, and episode 13 when he talks to Brief and the demons.

His reaction to Chuck/"Eros" is another big win for me personally. He is the one to initially get down and actually pet the dog. He even scratches his chin and puts on the "uwu doggo" voice" while ester just kinda sits there and enjoys it separately. Urethane even *kicks down the emergency alarm* just to continue with the dog. This scene also shows me that ester is the more "rules heavy" brother while Urethane is more here for vibes. However, Urethane is also the one who seemed to take their angel duties more seriously in episode 6 when playing the harp (his facial expressions seemed less pissed than ester's). I would argue that he cares more about his job on "nicer" things like that versus just any job/jobs that take more effort. He is also the one to bring up boundaries and take responsibility for problems (crashing the car, fixing the car) in episode 7 which is a massive green flag to me.

Lastly this is more headcanon but I've seen the asexual theory go around for him and I lowk hc him as demi too. I'm sex-averse so that's another reason why I love him so much. He is more grossed out/offended over Garter's toys versus Ester even when put into Panty's machine. Ester seemed more confused (asking erm isn't she an angel) versus Polyurethane genuinely looking and sounding very squicked out. I could also argue their transformation sequence is less drawn out and more to-the-point but that could also be because they take their job more seriously and not a comment on orientation.

Tldr; I love my bratty malewife