Dropping Benjen and adding:

Jarrett Frey is an imagining of one of Danwell Frey’s stillborn children if one had lived.

He is a Grandson of Waldar Frey and born in 272 AC and is about 28

He’s a knight and also scoured the seas as a pirate on and off and was a first mate before he went back to land shortly as his kin are being killed off (or all slaughtered in show verse)