Having items be unlocked by specific characters is fine, but it'd be better if you could see in game that they had an unlock requirement. Perhaps they always appear in the shop, but are grayed out with a "Reach x love with ???", with "???" being revealed when you meet that character. That way you know an item exists and if you can get it now, or if you have to go dwelling to find more.
On a related note, having some kind of vague idea of where to pick up characters is a good idea- Maybe your little AI having a 'search zone' for life signs?