The image also includes a note from the official artist. While it's entirely in Japanese, Tavi on Twitter was kind enough to provide a translation. I've reproduced this translation below for the sake of convenience. Thanks, Tavi!
I was thinking of something that's not too fashionable but also easier to wear. I came to the conclusion that he would like wearing something that hides his body (yknow the muscles) so that they wouldn't stand out. Keeps the pendant on because it allows Prevasion. Also please change the hair color back. o(^▽^)o'
Additionally, in the post where they shared this image, Inti Creates revealed that Ouka is two years older than GV, which essentially makes her 16. Pretty intriguing!