Ser Eddrick Lannett is the current castellan of Casterly Rock and belongs to an offshoot of the Lannister line.
Eddrick squired for Tyrius Lannister at the age of nine and has remained close with the Lannister House ever since.
Not much is known about Ser Eddrick's history, but in recent years he has been shown to be a fat, cowardly man, with a taste for drink. The man's shortcomings made his close ties with the late head of the Lannister household, Loren Lannister, even more surprising, but despite all odds Eddrick seemed to be Loren's closest confidant.
Ser Eddrick led the envoy sent to accompany Lady Aeslyn Targaryen to Casterly Rock, and took place in the ceremony of the exchanging of the cloaks during the Lady Targaryen and Damon Lannister's wedding.
Eddrick was raised to the position of castellan of Casterly Rock in Loren's absence as Hand of the King.
With the death of Loren Lannister at the beginning of the third era Ser Eddrick accompanied the wain meant for the lord's bones from Casterly Rock to King's Landing. Due to the Spring Without Sun the envoy was forced to first travel south along the Ocean Road and then north along the Roseroad, a voyage during which Ser Eddrick took ill, developing a bloody cough.
Severely ill, Ser Eddrick arrived at King's Landing where he waited on an audience with the King, unaware that Jeyne Lannister had failed to inform the monarch of his presence.
As luck would have it, the knight would accidentally run into the King in the following weeks, a meeting during which the knight disclosed Damon's true heritage.
"Eddrick Lannett. I'll never understand what Loren saw in that bumbling fool." - Jeyne Lannister
“I sent him back to Nunn’s Deep,” she explained.
“Because he’s a lackwit.”
“He’s been at Casterly Rock since before I was born,” Damon argued, appalled.