So I got curious and watched the first 10 episodes of this series on YT.
First, there's a lot of them, lol.
Second, Mirai and Heeko aren't the main characters (Hikaru is the MC and I guess Kuroi is like the second MC?). They appear for the first time on ep. 10, but seeing that episode got 5M views, well, I guess they were really popular and that's why there's a manga about them.
Truly an interesting turn of events.