Roobarb was a green dog, who lived on his own in a house (two up - two down).  He wasn't the most intelligent dog, but that didn't stop him from pretending to be.  He would come up with ideas and that usually meant him going to his shed at the bottom of his garden and making his invention become reality (which rarely worked). 

Roobard always had to look over his shoulder, because Custard was always around waiting to set him up.

Custard was a fat pink cat who lived next door to Roobarb.  He liked to laze around on the fence watching Roobarb come up with his inventions.  He would try to humiliate Roobarb with any chance he could and would normally be seen laughing the loudest at his misfortune (a bit of a bully really).

If Custard saw an opportunity that was favourable to himself then he would team up with Roobarb, but if it started to go wrong then he would immediately jump ship.

These were quite evil looking birds, that would also laugh alot at Roobarb.  These birds were really Roobarb's audience and he would show off his inventions or skills to them.  But as you've heard previously, Roobarb's plans rarely worked so the birds were mostly laughing.