Sesshomaru is a really elaborate and intricately-designed character. He looks like Rumiko Takahashi spends hours each time she draws him.
I get the feeling that Takahashi grows fond of her characters and loves them like as if they were her children after some time. I'd bet this is the reason why she's always so apprehensive about ending a series, and when they do end, that the main characters continue to make small appearances every now and then for other things: She just can't bear to part with them forever.