Much like a friend with benefits except the benefits go beyond the sexual sphere. The boy becomes attached and wants both emotional and sexual support from you without any committment. If someone asks, much like friends with benefits, you are not together, just friends. You know, friends who call each other just to say good night, friends who invite each other for holidays, friends who sleepover once a week, friends who have sex and do all other things sexual yet listen to sappy songs in the dark, etc. etc. Really for all intents and purposes you are a couple, but refuse to admit it and pretend everyone else doesn't know it.
Pseudo-boyfriend, "So you wanna hang out tonight." girl, "Yeah sure." P.B, "Well I am hungry so how about we get dinner, maybe go see a movie later." girl, "is it ok if my roommate comes?" p.b, "I just wanted it to be us tonight, I really need to talk to you about some things that have been bothering me and maybe you could stay over tonight" girl,"ok see ya in a bit." Roommate, "so what you up to tonight?" girl, "Just hanging out with __________." Roommate, " right...."