Affection and Gift guide:
Check a character's Affection by going into Status Menu. Affection is changed by doing things (like helping) and giving gifts. A gift can raise affection +1, or +2. Characters also have gifts they hate, and offering one will change their affection by -2 points (Affection can't go negative, at least). Save before offering a gift if you're not sure!
Getting gifts: The white-haired guy at school will give you lots of gifts if you talk to him. He's located in the office where Mizuho works during the day, at the end of the school. Other gifts: Ribbons appear in the early game, and the waffle box is in Fuyu, the town you can take a bus trip to.
Here's a short list of some things characters like:
Mizuho: Stylish Sunglasses, Expensive Pen, Black Tights. +2 points affection when you help her at school. +1 for Pizza. -2 points for offering Plum Brandy at school, which is bad!
Sigrun: Pretty Ribbon, BunBun Chan Phone Accessory, BunBun Chan Plushie, Mobage gift card, Tabloid Magazine. Dislikes peaches according to Mizuho. Once you get over 30 affection with Sigrun, find her at the castle entrance one day. She'll be moody for a while after that.
Suzuya: The maid who comes once a week. Marbled steak, black tights, peach, coconut oil, sakura handkerchief, beautiful fan, pizza, box of chocolates, floral arrangement, sweet perfume.