I'll contribute for the lulz.
Rumia: You'll never see her, literally.
Cirno: You'll probably freeze to death.
Meiling: She'll just sleep all the time.
Patchouli: Medical bills piling up, and the fact that she'll always be reading.
Koakuma: Its not like she'll care about you. Too busy care taking Patchy.
Sakuya: Same for Koakuma but for Remilia.
Remilia: You better supply lots of blood.
Flandre: She'll ask you to play with her, it's all over.
Letty: Same for Cirno. And you won't see her all around the year but Winter.
Chen: For one, she's an animal and thats against the law. In Youkai years, she's a little girl. And then there's Ran.
Alice: She'll be to busy fantasizing about Marisa.
Any Prismriver: They'll be too busy making sounds and bleeding your ears to death. Merlin will probably go insane on you.
Youmu: Same for Sakuya and Koakuma but with Yuyuko.
Yuyuko: You better be rich, because you'll have to buy food every single day.
Ran: Same for Youmu, but with Yukari and Chen. And the fact that she's an animal.
Yukari: ZZZZ.....Zzzz...ZZZZZ Sorry, what was that?
Wriggle: People who first see Wriggle will probably think you're a homosexual, and there'll be bugs everywhere. No toilet jokes, soir.
Mystia: You'll go deaf after her songs.
Keine: Caving, make her mad and you're history, Kinda. you'll just lose your history of being born. And Mokou fantasizing.
Tewi: Silly rabbit, Tricks are for kids!
Reisen: Make her mad and you'll go insane within minutes.
Eirin: Too busy with NEET.
Kaguya: Too busy with video games. You can play with her, but she'll only consider you an opponent.
Mokou: Too busy raging at Kaguya.
Yuka: Don't touch her flowers. And you'll probably become her punching bag.
Medicince: She's a doll, seriously guys. And she looks like a little girl. Pedophilles.
Komachi: Same for Yukari. And getting preached to by Shikieiki
Shikieiki: She'll know everything you're doing, so you better be a good person at heart. And the fact that she's a short stuff. Pedophillia, once again.
Aya: She'll take pics of you. Put them in the newspaper and brag. And she's an animal.
Minoriko: She smells nice, but she'll think you want to eat her, and she'll attack. Pedophillia again.
Shizuha: ...I dunna know. Pedophilia too, I guess.
Hina: You'll be dizzy all the time from her spinning. Though you'll be less unfortunate.
Nitori: Kappa rumors. And she'll always be making things. And will easily be startled by you and run away.
Momiji: Too busy with Aya fantasizing. Animals, again.
Sanae: Better watch out for that old ha- Er, young goddess Kanako.
Kanako: Better make her feel young.
Sawuko: Pedophillia, I guess you can count her as an animal.
Kisume: ......Buckets. Probably Pedophilia.
Yamame: Spiders. My greatest fear. And she'll probably eat your head after mating.
Parsee: You better act jealous of things she can do or she'll rage at you.
Yuugi: Intercourse will hurt. She's a deadly drunk. And you better run when she's mad.
Satori: "You've been playing Super Mario Sunshine, haven't you?" Also, pedophillia.
Orin: Animals, your corpse. Need I say more?
Okuu: Animals, death of nuclear radiation. Either that or she'll make you Atom Smash with her.
Koishi: Without the Satori thing, Pedophillia.
Nazrin: Watch out for Shou. Pedophillia. And she'll bite you when you're in the kokatsu. And, FINDERS KEEPERS LOSERS WEEPERS!!!
Kogasa: Everytime she surprises you. Be surprised. Possibly her umbrella will freak you out at times I guess. And if its nessecary, say UR DOING IT WRONG TO HER then yell DESU DESU DESU.
Ichirin: Unzan will always be watching you. Always.
Murasa: Make her mad and she'll drown you in a puddle. Plus she's a ghost, seriously. And her Anchors.
Shou: Too busy with Nazrin.
Byakuren: She'll make you go to church everyday with her. And you'll help preach.
Nue: She'll probably surprise you like Kogasa, not like it'll work. But be surprised anyway.
(For ladies)
Rinnosuke: He'll live longer than you anyway. And he'll be selling things all the time.
SPOILERS
Hatate: She'll be a 15 year old school girl according to how she talks in Double Spoiler.
Reimu: Better have some money.
Marisa: Too busy stealing things and eating mushrooms.