He is one of the two former NCR1st Recon snipers who are now sentries in Novac. He and Craig Boone alternate shifts in Dinky the T-Rex's mouth. Born in North Vegas, Manny grew up with his cousins, and eventually joined the Great Khans with them. Then "something happened" (Manny doesn't specify what) and he decided to leave. He enlisted in the NCR military and was recruited into 1st Recon, becoming Boone's spotter and best friend.
Being a former member of the Great Khans, he made it a point to be absent when 1st Recon was deployed to rout out the gang at Bitter Springs. Even though he wasn't at the battle of Bitter Springs and harbors no ill will towards the NCR, it was the reason he didn't re-enlist; if asked, he says "it did take something out of it for me. Just wasn't the same." After retiring, he convinced Boone and his wife to move to Novac. Carla hated Novac, and she and Manny didn't see eye-to-eye on anything. When Carla disappeared, Manny was the first person Boone told, but Boone ended their friendship after seeing that Manny was glad.
Boone is likely unaware that Manny has certain feelings for him, which is implied through a jealous comment he makes about Carla if the player has the Confirmed Bachelor or Cherchez La Femme perk.