Reigen Arataka 

For you: Dollface, sweetcheeks, sweet thing, babe
Reigen's nicknames are often sleazy and repeated as a casual reoccurrence. He says them in public, and at home. He has no shame.

For him: Con man, sexy, pretty boy, hot stuff, love muffin, heartthrob

Your nicknames for him are just to boost his ego, and occasionally get him flustered. Don't take these nicknames seriously, they're just to poke fun at him!

Teruki Hanazawa

For you: Babe
Teruki keeps it simple. If he doesn't call you babe, then it'll just be your name. He will call you babe whenever and wherever, public, in front of friends, etc. He's not afraid to show you off.

For him: Teru
Simple, simple, simple! There isn't much of a need for you guy to shower one another with silly little names. 

Ritsu Kageyama 

For you: Darling, precious 
Ritsu calling you pet names is a pretty rare occurrence, but when he does it'll be just you two together. He's a bit down low on the public affection. He'll get more used to when he gets older.

For him: handsome, power boy

You called him power boy ONCE, he didn't seem to like it very much, but you just wanted him to know you thought he was strong. Ritsu thought it was somewhat cute, but a little strange.

Shigeo Kageyama

For you: Angel
Mob doesn't even think about calling you by nicknames. But once, he called you angel, and he got very flustered by it. You told him how sweet it was but he avoided doing it again. Once he gets a bit older and more mature, he will do it more.

For him: Shige, softy, love
You avoid calling him by pet names because it often gets him flushed. But as you two get older, it becomes a more comfortable reoccurrence. 

Shou Suzuki

For you: Babe, sweetie, baby, eye candy

Shou is scandalous with his pet names. He'll often use them to course you to doing something, or to fluster you when you're being pouty.

For him: Babe, handsome, bad boy
These pet names are also to poke fun at Shou, he doesn't mind them though. It indeed boosts his ego.

Dimple

For you: Love, beautiful, dear, doll, hun, toots
Surprisingly, Dimple is rather seductive (when he's got the right body) so these pet names slip right out of his mouth with ease. He'll use the more affectionate name in private, but in public, he'll refer to you as toots or hun. 

For him: Rosy cheeks, good-looking, boo
Dimple isn't one to approach you while he's got no body. So you don't exactly know about his ghostly side. Which is pretty ironic considering you call him boo.