I was hesitant to post this one, given what’s going on. But, she’s part of Megumi’s friends and I can’t ignore that.
Sickness is a constant in any universe and none can escape it. Yes, an excellent immune system can stave it off, but even the best immune system can be caught off guard. Pestilence serves to remind us of that fact.
Personality-wise, think Shockwave, but with ethics and a moral code. She’s the scientist among the Four Horsemen, her specialty being biology. Even she can see the logic in ethics and emotions. It was those factors that let her obtain the Languorem Ring to become Kamen Rider Pestilence.
She helps War, Famine, and Death to train Lacey Thanatos to be Kamen Rider Apocalypse to help Megumi Hishikawa stop Lord Vortech and Megumi’s father, Hiro Adachi.