I couldn't think of what to do, so I just went for this because everyone else does it.
HfY = His for you
YfH = Yours for him

Loki:
I'd just like to publicly thank you for your monumental efforts in persuading Loki to purchase a phone. He even paid for it.

HfY: My queen
YfH: That idiot with the cow horns and glowstick of destiny

Tony:

HfY: Honey
YfH: Some idiot

Steve:

HfY: Like Steve knows how to change the contact name on his phone.
YfH: My gentleman 😍

Clint:

HfY: Y/N/N
YfH: Katniss Everdeen

Bruce:

HfY: Geek
YfH: Nerd

Thor:
Yeah... He doesn't have a phone.

Pietro:

HfY: Anger Management Issues
YfH: The Flash (He hates being compared to the "other speedy guy")

Bucky:

HfY: Beautiful
YfH: Good guy