Head Cannon
I define "head cannon" as all the things that are up to the reader to decide. This is where I post what I think makes Greed tick. The following paragraphs don't have much to do with each other. Rather, each one is a separate thought.
I'm not going to get into his rebellion and departure too much. I'm saving that for a manga. I told the whole story to a certain somebody (you know who you are) and it took me an hour just to type out the basic storyline. I'm going over the script in my mind. What I will reveal is that Greed used to be one of the homunculi who stirred up chaos in various cities. Before that, he was assigned to guard Father's lair.
Greed wears his Shield somewhat like clothing. If you notice, his "shirt" transforms with him when he takes on his Ultimate Shield form. I think he uses it as a body suit to protect his internal organs.
I don't think any of the homunculi really get along with each other, save for Lust and Gluttony. I get the feeling that by the way that Greed trash-talks his siblings, he doesn't particularly like any of them, but he tolerates some more than others. I'd think that Lust is probably the boss when they go on assignments, and Greed doesn't like being ordered around. Gluttony's constant complaining of hunger gets on Greed's nerves. I'm sure that Greed and Envy have had it in for each other pretty much since birth. While Sloth doesn't particularly bother anyone, he doesn't really do anything much in the first place, so Greed just dismisses him as lazy. Greed once mentioned that Pride gives him the creeps, but then again, Pride probably gives all his siblings the creeps. And of course, we all know what Greed thinks of Wrath.
Although I'm sure he has blood on his hands, I don't think Greed enjoys killing humans.
I still haven't figured out why Greed won't lie. He seems perfectly capable of doing it with a straight face. I just don't know what made him resolve never to lie in the first place. Also, do half-truths count as lies according to him?
Greed hates fighting women because he sees them as things of beauty. Kinda like one would view fine china. You wouldn't want to break it.
When Greed infiltrated the lab that contained all the chimeras, he probably made some bold statement about how they'd owe him their lives if he released them. Even then, the chimeras didn't put up much of a fuss. To them anything would be better than being cooped up in a lab.
Some people like to ship Greed with Martel. Personally, I can't see Greed with anybody. Although Martel is female, I think the whole gang considers her as "one of the boys".
I have no idea what Greed and the gang do all day at the Devil's Nest. I mean, do they live there? I guess so, but where?
Feel free to chime in with your own two cents on any of these topics.