If you are staying in Front of Ioris House, there are trees on the right Side of the House. There he is.

I’ve never seen him. To the right of Lori’s house is Ralph’s cabin, Help….

Can you marry him? Is there any secret marriage with him like there has been in other games?

He seems to really enjoy the Mugwort Daifuku. I gave him one just to clear out my inventory and he was doing the excited/happy emote characters do. "Th-This is--!" I would definitely say it's one of his loved gifts.

he loves any quality grilled fish and will thank you for it .

he will be next to Iori's house every day apart from Rainy weather and severe weather conditions like blizzards

Matsuyuki loves the Shiny Watch, declaring “This is-!” and gets very excited to it. I haven’t tried theGrilled Fish yet, but will try that next. He doesn’t seem to care for leeks... don’t try that on him. I think it’s either a Neutral or a Dislike.