Blip is Jan and Jayce's pet monkey. He is probably their pet to help them cope with the death of their parents. Blip was not in the 2005 miniseries in which Jan and Jayce were about 8 or 9 but he was in the original Space ghost cartoon series from the very beginning where Jan and Jayce are around 13-15. They must have got him when Jan and Jayce were between 10 and 12. Blip wears an eye mask and costume that is identical to the ones that Jayce and jan wear (which include jet-packs.)
Blip usually slips away and turns invisible when Jan and Jayce get captured, to either get Space Ghost or find some other way to help Jayce and Jan.