In the 5th volume of the Yami no Matsuei manga, it was revealed that Tatsumi was Tsuzuki’s third partner, but their partnership only lasted three months until Tatsumi quit. Tsuzuki’s emotional breakdowns reminded Tasumi too much of his mother, and he still blamed himself over her death. Although their relationship was partially resolved in volume 5, Tatsumi remains uncertain and overprotective of Tsuzuki, as he felt guilty over the end of their partnership.
If I remember correctly, Tsuzuki often asks Tatsumi to make him snacks, and even though Tatsumi complains about it, he makes them anyway! Doesn’t Kuromitsu (黒蜜) make a lovely Tsuzuki? I don’t know who Tatsumi’s cosplayer is, but s/he is equally lovely as well! Thanks to Claro for sending this in!