Yuuichi is initially hostile to taking in Makoto Sawatari. Because she has amnesia, she needs a name. His first suggestion is Satsumura Kyouko (Kill-Village-Disaster-Child).
In Nayuki's route of the visual novel Yuuichi gets creative with how to make sure she's awake by asking her a series of general questions, then he tops it all off by asking her measurements. She goes as far as "Breast eighty th..." before she realizes what she's saying and wakes up completely.
Being the Deadpan Snarker than he is, Yuuichi has quite a few of these, especially when he's countering all of Makoto's pranks. He does everything from jumping up and screaming "ZERO!" as she counts down to the execution of a prank to arranging his furniture to trap her in his closet where she was waiting for him to go to sleep.
On Mai's route, you get the following choice:
Try touching her breasts.
Try not acting like a total imbecile.
When figuring out how to get Mai back into school, one of the possible plans is to use Sayuri's sexual charms on the headmaster. If you select that, you get an imagine spot where she seduces the headmaster, but you decide not to mention the plan. You can then select the plan of using your own sexual charms on the headmaster, which gives a nearly identical imagine spot with you in Sayuri's place.
In the anime Ayu trying to Glomp Yuuichi, Yuuichi getting out of the way and Ayu crashing into a tree instead.
Yuichi is curious to the extent of Mai's passivity at lunch and tries getting her to talk. She barely introduces herself. Then she wants something from his lunchbox and he passes it over. Then he grabs all the lunchboxes and puts them out of her reach so that she has to ask him for everything. And gets the exact same response as before, except now she asks for a sausage instead of a bit of omelette.