I have a whole bunch of Marvel characters but the biggest one is going to be Sam Wilson, who is... not as Steve knows him. He's from a shitty world where everyone is terrible. He's a genius badass scientist who was a consultant for SHIELD (or for Fury, really), basically married a genderqueer version of Vision who was an all-knowing space robot until she got thwomped by Galactus, and has a lot of Walls. He's BFFs with Tony, runs the medical clinic here and has been around since day one so he's seen about six Steves come and go.
I also have Warp World's Laura Kinney (Weapon Hex), who is a mashup of X-23 and Scarlet Witch and helped cofound the Defenders. She's sort of second-in-command/co-leader with Soldier Supreme, who's Steve and Dr Strange combined. She started out mostly mingling with the Avengers characters and dated James Rogers for a while, but these days Tony and to some extent Bruce are the main ones she still talks to unless you count Riri who she's really good friends with along with Hugo Weasley. She's about the same age as Kate too.
Plus I have three other 616 characters: Storm, Thor and Loki's older sister Angela, and Aracely who is a tiny psychic Mexica with an Aztec war god in her head. She only stops talking when she's eating, which is quite a lot. The codename Hummingbird is extremely apt.