I remember quite a long time ago a cartoon which I’ve forgotten the name of, I saw it on ABC in Australia between the ages of 6 to 8, I’m currently 24.
The main character's name is Leo I think and he has a girlfriend named Diane, in Leo's room his cupboard apparently connects to other dimensions, most of which are dangerous, apparently there's this monster who is always visiting him who Leo is friendly with.
Leo's neighbor is apparently suspicious of him and his entire family, in one episode she's investigating his room and goes into the cupboard to fall into another dimension where Leo and this monster have to rescue her, Leo's neighbor also sings the Opening (which Leo and Diane are named in).
In one episode something happens to the dimensions which cause them to filter out effecting the entire house, once scene in the main hall with the stairs the floor becomes lava and Leo's parents have to use spelunking gear to climb along the ceiling to get to the living room, as they are doing this Leo's aunt enters from the front door and while his parents try and warn her about the lava she steps into the hall and everything is apparently normal.
Apparently Leo goes to great length to hide his monster friend and his cupboard from everyone resulting in situations where Leo ditches Diane or starts acting weird around her.