I wonder who that guy is! people in the previous thread link his appearance to the largest image in the montage of her coming out of love hotels.
I've also noticed this lol. If it's the same guy, he's been shown in Ch.1 p.14 and Ch.15 p.14, with the largest montage panel both times (as you mentioned).
My prediction is that this new character is probably one of Agawa's favorite sex friends (like her childhood friend Yuki), and like Yuki in the OG manga, will be one of the main romantic rivals for Gotou. He even got larger montage panels than Yuki, which is interesting since Takeda Super hasn't really explored Agawa's relationship with Yuki in this series just yet. (Though perhaps he doesn't intend to, since the OG series has Yuki as the main rival, and Takeda Super might want to switch things up for this series.)
Another possibility is that he's (also) Agawa's middle school crush, who Shiina slept with in front of Agawa (Ch.29 p.11 and Vol. 4's revised Ch.31 p.7). If so, he and Agawa might share a more complicated past than her other sex friends, and Agawa might still harbor some romantic feelings for him (and he might feel similarly towards her as well).
To go even further, I think the new guy could be a toxic "ex-boyfriend" type who, like Yuki and Gotou, might want to have Agawa all to himself. Agawa might hate his guts but also be extremely sexually attracted to him, which could lead to some erotic scenarios due to manipulation and coercion.
With the Kiyomi arc now over (or at least, I hope so lol), it seems like Takeda Super has got some major plot development in store for Agawa and Gotou. Which is great, since sex scenes alone isn't what makes this series so erotic, and it's the combination of character progression and sex scenes that create the many peak moments this manga has had.
Thanks
@Chekov for the chapter!