Seiryu:
New here. Have to say Only person I like as much as Ataru is Lum. There's something about how just innocent she can be that appeals to me as a character. I also like that she has no qualms when it comes to going to great lengths to get what she wants out of her Darling. Doesn't hurt that she goes through development too.
Megane:
Well, of course it's Lum. She's not just cute and sexy (there are plenty of cute, sexy girls in UY) but she's sweet, loving and devoted. How can anyone not like her? Even Ataru's parents like her, and AMAZINGLY raise absolutely no objection to Lum living in Ataru's room in their house!! Although there are times when I wonder if that has less to do with them liking Lum than with not caring anything at all about their son in the least (Mrs. Moroboshi: "Oh, how I wish I'd never had him!").
Everyone wonders why Lum so completely and obsessively attached herself to Ataru, and I have to think it's because she's an Oni. Oni are supposed to be kind of like 'bad karma demons', so it's Ataru's own bad karma that attracts her like a magnet. And indeed, complications ensue due to all of Lum's friends that cause trouble for Ataru, so it's the wheel of karma -- "what goes around, comes around" or to put it in Judeo-Christian terms "as you sow, so shall you reap". Spread bad karma and it will return to bite you in the butt.
Yet Lum herself is the best thing that ever happened to Ataru, even though he doesn't realize it (or at least refuses to admit it to himself). Everyone else knows it immediately though, and is envious of Ataru's status in Lum's eyes. Especially Mendou and the so-called "Stormtroopers", led by Megane. They all have a keen appreciation of Lum, and a consequent resentment of Ataru for having the undeserved good luck to be the object of Lum's affection.