May

Season 1
She is a cheerful cat/human courier, and whenever she's called, she will come running and appear all of a sudden. One day while delivering mail, she saw Joker who was running away from Ruri and Joker pretended to be May's boyfriend in an alley. After that May became a big fan of Joker chasing after him and hated Ruri who was trying to put Joker in prison to receive a bounty.

She makes appearances in season 1 where she appears as soon and delivers letters.

Season 4
After the catastrophic events due to the war, May was worried about Joker's whereabouts. She heard that Joker was alive and decided to start looking for him due to her love for him.

Trivia

 * Her awakening was announced in December and said to be released in April. Her awakening indeed came out April, 2019.
 * She's a cat girl like Daisy and Chloe. Catty is technically full human who was raised by cats.

Season 1

 * Joker: Unrequitted love
 * Ruri: "Joker's mine!"