The character drafts for Reo/Leo and Mabu were done by Migi ( ミギー ). They seem to be active with doujin work (they’re the head of their 天羅万象 circle) and they have done some previous professional work doing cover art and illustrations for various things (nothing that I recognize, unfortunately). You can check out their blog or Pixiv for more. They work mainly with watercolours and have painted some really beautiful artworks!
The spin-off manga serialisation will be published in Rutile and Rutile SWEET, both in a magazine and online. They publish exclusively BL. I’m not really sure if I can make more generalisations or guesses based on who the publisher is (this is not my area of expertise), but looking through what they’ve published I can spot one of my favourites within the genre so yay for that!
The serialisation will be done by Misaki Saitō ( 斎藤岬 ). She’s a mangaka and illustrator who debuted in 1995 and has been drawing manga for a more mature audience ever since. She appears to be drawing mystery, horror, supernatural and historical manga. You can check out her manga from her website. Her latest work is Keishichou Battouka. Interestingly she also has a tumblr blog, although not an active one.
The logo was made by Wataru Osakabe ( 越阪部ワタル ). He’s an art director and graphic designer who represents Lovedesign co. He has previously worked on other Ikuhara projects such as Mawaru Penguindrum and Yuri Kuma Arashi, as well as more recently on Doukyuusei, Inuyashiki and Koi wa Ameagari no You ni. You can check their blog for more.