Not much is known about Robbie himself as his only purpose in the games he appears in is to unsettle and frighten the player.
The first time I encountered Robbie was on TEKTEK.ORG the iconic GAIA ONLINE fansite.THIS custom profile made by user HIKARI KITKIT was my first glimpse of the iconic rabbit. Indeed, Robbie was catnip to the edgey tweens and teens of the 2000s who pinned his image on their MYSPACE pages and GAIA profiles.
Big thanks to the SILENT HILL WIKI for being such a great source.