Venom is a parasite, he was once with Eddie Brock but left him and is now with Y/N Stark. No one knows she has venom, so she's still an Avenger but with venom. Half the time venom don't come out, so only Y/N is the only one to hear him unless he comes out for people to see.

Y/N was in the lab and working, not noticing that team watching her from the doorway. She's talking to venom, but the team can't hear him talking.