One of the most profound serious moments (to me) is in ch108, not for the subtext but for the honest development that some may overlook in favor of buttsex, I mean subtext.
Kaneki wasn’t in the mood for visitors and told him as much. Tsukiyama could have responded with humour, or with anger, or something otherwise dramatic, or simply left, but instead, gently pressed forward. He hasn’t been one to argue with or cause trouble for Kaneki lately, so it was a pretty unusual and risky move to just barge in.
He presents Kaneki with some books, which he’s said (earlier in the manga) helped him get through some rough times. He’s trying again to establish a connection that he had tarnished so many months ago, in their first two dates. He’s taking the remnants from that —hey, we’re both book nerds— and building it into something more constructive, more positive. In short, he’s correcting a mistake.
Not only that, he does something none of the other characters have seemingly done yet— he sits down with Kaneki and tries to reassure him. Kaneki feels bad for what he did, but Tsukiyama insists he’s still a good guy….he’s literally the first, possibly only (in the manga), to say “Kaneki did nothing wrong”. He remains non-judgmental of Kaneki’s actions, even though he’d been hurt too. He tells Kaneki not to be too hard on himself. Shit happens. And if Kaneki’s changing, he wants to change with him. He insists he’ll stay by his side and will shift his role to be even better for him.
All this from a guy who can’t be serious for two seconds to save his life, and he actually got through one scene without any jokes (even though he dipped into some dramatic posing) and without being unreasonable. it’s quite a turning point in their relationship. Quite an accomplishment in his character.