Random factoids that might be too technical for the Wiki page...

• As far as I can tell, discipline does not affect growth at all. However, the Yasashii tracks care mistakes that are made when a discipline call is active vs. when one is not active. Because I saw two counters in the growth chart logic, I assumed one was care mistakes and the other one was discipline mistakes. However, this does not appear to be the case.
• Sickness is partly RNG-based, but the Yasashii will not get sick at all if you have fed it less than 4 candies. And also if you have fewer than 3 care mistakes (that did not occur during a discipline call).
• The number of candies fed to the Yasashii resets when it gets sick, so if you feed a bunch of candy, you basically only get punished for it once.
• Sickness only occurs once a day and the flag that tracks whether the Yasashii has gotten sick today resets when the Yasashii wakes up. Same with discipline.
• When the hunger timer is being reset, if it's the afternoon, all adult characters will take an extra 10 minutes for the hungry heart to drop. Kabuto Oyajitchi and Michelletchi take an extra 20 minutes.
• Discipline is also partly RNG-based, but the Yasashii does not call for discipline if it has < 3 happy hearts. It also becomes less likely to call for discipline as the meter fills up (so it will take longer for the call to occur)
• 4 sicknesses in a single stage kills the Yasashii (although I guess they don't die in this one, from a lore perspective). I got this wrong initially, but yes. The Yasashii can get sick 3 time, then the 4th time kills it.
• Like other devices, 8 poops on a screen causes the Yasashii to get sick immediately. However, Yasashii is unique in that if you have let 3 poops stack up, that registers as a care mistake.
• The Yasashii is also very unique in that you apparently only have 132 seconds to respond to a call for attention, a sickness, or a poop before a care mistake is registered, not to mention the RNG used in the growth chart... but this is all critical information and is on the Fandom Wiki page now. I speculate that maybe they originally meant the timer to be 5 minutes, because the care mistake countdown is 0x12c which, if interpreted as hex, would be a nice round 300. Hell, maybe they even originally intended the countdown to be 5 hours (300 minutes) which would be much more やさしい, but maybe they thought that made the Yasashii too easy.
• On evolution, the time spent in the new stage is calculated as (in minutes): random number between 0-9 + 10 * current tens of minutes on clock + 60 * character-specific value. This is similar to Mothra but not as RNG-driven (Mothra could have a variance of 16 hours, Yasashii can only vary by 10 minutes)
• Yasashiis have a set lifespan (aside from the slight RNG used to determine how long the stage lasts). When adults reach their time to evolution, they just leave immediately. The same flag that determines a good ending on other devices (controlled by number of care mistakes) is also set here... but the care mistakes are actually reset to 0 before checking them, which means the good ending flag is ALWAYS set regardless of the care you took. If you force the good ending flag to NOT be set in emulator, the UFO doesn't leave an egg behind when it flies away.
• The Yasashii has a debug fast-forward mode that is likely activated by some jumpers on the board. I found it by accident because in emulation, the device was ticking up 1 minute for every second, and I had no idea why. I'm not sure what all it does yet, but it definitely makes both the clock minutes and also the minutes to evolution go up by 1 every second.
• The time to poop is a character-specific stat which is unique to the Yasashii (at least among all the devices I've studied). I guess it makes sense since tracking poop time is so important to getting good characters on the Yasashii.
• The timers that control evolution, pooping, and when hearts drop halt under various conditions - probably any time you are not on the main screen or on the clock screen, the timers are halted. My personal guess is that this is because e.g. you don't want a call for attention to occur during a game, because that would be extremely confusing (you could see a button light up that didn't actually do anything for the game)
• There is a bug with the game, where if it ends on you revealing a cake under a box (whether you quit or let the game finish), the timers for pooping and happy/hungry hearts decrease by 1. So if you want to predict your Yasashii's poop times accurately, you need to track this.
• The bug, combined with the fact that the Yasashii can't get sick from too many candies as long as the number of care mistakes is < 3, suggests that the optimal strat is just to feed candies (no games at all) and make sure to catch the poop times exactly. This way there is no annoying, hard-to-track variance in the poop times.
• Once a timer-based care mistake has occurred, the code stops the timer from going off again unless all conditions that could cause a care mistake have been cleared. So I'm wondering if it's possible that you could leave happy hearts empty forever and it would only register the 1 care mistake. There is code that seems to WANT to allow the timer to go off again (they can get set up again if you don't address all the conditions that could cause a care mistake), but in emulation I have not yet been able to make it work. The timer never starts counting down again, and I'm not sure what can make it count down again other than addressing all the conditions that cause a care mistake. Very weird.
• (It is not advisable to leave hungry hearts empty, or let it be sick forever, because if left like that for 16 hours it leaves)
• I think the game has a 50% probability to win on any round. It randomly rolls which item to display to you, and then you choose a button, and it randomly rolls which item should be under the box you chose. It is not like, distributing items under the boxes or anything. The Tama code is always doing the dumbest, cheapest thing under the hood lol
• A few times during my perfect care run, I've had poop not occur when I expected. My suspicion is that this is happening because when the poop timer elapses, the poop doesn't happen right away - it effectively gets scheduled and then happens later. (note that this doesn't happen if you're on the clock screen - it just poops immediately in the background) But some actions, like entering or leaving the clock screen, or opening or leaving the menu, can effectively cancel the poop from occurring. They basically just overwrite whatever is currently scheduled. As far as I can tell, this ONLY affects poop because ONLY poop gets scheduled like this.
• The poop code in the Yasashii is duplicated and occurs in 2 separate places. If you have the clock open, one version of the poop code runs. Otherwise the other version runs. They appear to behave identically (i.e. both will cause a care mistake for >= 3 poops, both will cause sickness / death for 8 poops), I just thought it was interesting that it's duplicated.
• I was surprised when a call for attention occurred when Mametchi's Younger Brother pooped. At first I thought a discipline call had been canceled by poop (which looked impossible from the code). But after inspecting the code again, I believe that poop and sickness cause a call for attention for Oshamatchi, Mametchi's Younger Brother, Pantarotchi, and Ginjirotchi's Younger Brother. Other characters do not call for attention in that situation.