Ask Reldo about Robert the Strong and the dialogue will describe him a bit

Two Quest Points: Fast Quest Walkthrough: Talk to Unferth, Talk to Hild, Give her 5 Death Runes, Choose to buy boi gold 'Open' your Catspeak Amulet and use it to find Bob the Cat.

Talk to Bob, Talk to Gertrude, Talk to Reldo, the Varrock Castle Librarian, Find Bob and talk to him again.  

Talk to the Sphinx in Sophanem, Go to Unferth's House after the cut-scene, and talk to him, Do the chores on the chore list.

Talk to Unferth, Talk to the Apothecary in Varrock, Wearing the Doctor or Nurse Head equipment and White Robes, talk to Unferth and give him a Vial of Water, Find Bob and talk to him again.

Ask Reldo about Robert the Strong and the dialogue will describe him a bit. Then your cat will say Robert the Strong is Bob. Go back to Bob he will say he knows nothing of his Robert the Strong and then your cat says we must go to the Sphinx.

Talk to the Sphinx from the Icthlarin's Little Helper Quest, located in Sophanem. He will tell you that he too has noticed something about Bob, and then you have the option of either viewing a cut scene (around 5 minutes long), or just reading a summary of it.