When thinking back on the show I think I realize that my memory wasn't the greatest, and I may have forgotten/mixed-up some of the themes or happenings in my head (Usually tends to happen with loads of shows/movies before I watch them a second time), so it may be a bit wise to take what I say below with a pinch of salt, because I very much likely may come across as ignorant, or plain wrong!

The reason I love him so much is that you gradually, but surely realize that he is by human standards (This is important, I'll get to it later) an absolute, manipulative jerk. He'll trick young girls into becoming magical girls for what he believes is benefiting the universe, but also destroying them in the process. However during all this the main cast will seemingly still keep him around (More like tolerate him), because if you pay attention closely that while he is a dickbag, he is not an openly hostile entity.

Many times characters can still converse with him, and still get straight answers out of him even after he's revealed his true colors. The way this was done for me I felt was a very organic way of handling when his character appears in the main cast, and flawlessly integrated what arguably is the series main antagonist amongst the main cast.

Another thing I love about him is that while he displays some typical big baddy traits such as "Not understanding friendship, or human values" a twist is added where he isn't just questioning his confusion over these values as sarcasm, or being a snarker. From where his species comes from, he GENUINELY doesn't know why what everyone is going on about, or why they care so much.