When they were younger (like toddler age) Tsukasa and Saki would take baths together almost every night, and would complain if for some reason they couldn't. They really liked playing with the bubbles and toys put in there, and it was almost just an extension of their playtime together in their eyes. This concept especially helped young Tsukasa who was very opposed to baths at first, but he seemed to enjoy them more when Saki was with him.
These baths started to become less frequent as Saki started to visit the hospital more and their parents started getting worried that Saki could overheat in said baths. When Saki could stay the night at her own house, Tsukasa would prepare the bath with all these bubbles and toys and almost throw a big welcome home surprise for her, which Saki absolutely adored. However, at a certain point when they were both approaching middle school and Saki was staying overnight at the hospital more often, their parents put a stop to these baths, saying that they were too old for them, and since the two weren't even sleeping in the same place most the time, the two started to forget about them and moved along with their lives without it.
Flash forward to high school, where Saki lives in the Tenma household almost every night, the two were looking through a photo album when they were toddlers and found a photo their parents took of them bathing together. Tsukasa and Saki both reminisced on how much fun they had together, and Tsukasa blurted out "why can't we have another bath together?" At first he took it back and apologized for saying that, but Saki agreed that it would be nice to bathe like they used to. Then, they decided they would take a bath together again that night, and they would end up making it a routine, although it became monthly-ish instead of daily like back then.
Their first couple baths they both wore swimsuits because they were both like "we're in highschool now, it would be weird to bathe naked with each other!" Other than that, not much else changed from when they were younger. They still played with bubbles, they dug out some of their old bath toys and purchased a couple of replacements, and just had a lot of fun.
At a certain point their baths started to also become a way to vent and comfort each other. One night, on a bath night, Saki was having a bad day but didn't want it to carry over to their bath night fun. She asked Tsukasa to postpone it until tomorrow, but he insisted on having it and asked Saki to tell him what was wrong while in the bath. She agreed and vented to him about her day in the bath, which actually helped her calm down a significant amount. Now, they have their scheduled fun bath days where they just play around, and their unscheduled sad bath days where they talk to each other about what's on their mind when they need to.
One night, it was Tsukasa who was having a bad day and he called a sad bath day. Unfortunately, they had a fun bath day the day before and their swimsuits were in the wash. Even though he was in a bad mood, Tsukasa was going to call it off for another day, until Saki suggested nervously to just take a bath without their clothes. She felt weird about suggesting that, for the same reason why they implemented the swimsuit in the first place, and Tsukasa also felt weird about it upon suggestion, but they both agreed it was something they could try out and if they hated it, then they would never again do it.
So then they sat together in the bath, completely naked and kinda covering themselves up because they didn't want to be fully exposed. At first they didn't say anything, but eventually Tsukasa stated talking about what was on his mind like for the usual sad baths. He felt that venting while they were like this was easier and felt more personal. Likewise, Saki felt more connected to what Tsukasa was saying and it overall felt more personal too. They don't fully know why, but they both agreed that on sad bath days they would bathe nude and on fun bath days they would keep the swimsuit.
(feel free to interpret their relationship how you want, but my intention was strictly platonic siblings in case mod or anyone else reading this was curious)
