A while ago I replied in a thread about Belial, including everything I know to date about him – with the software changes, some links in that got broken, so I’ve just fixed those, made a few edits to bring it up-to-date, and am making this its own thread because I just recently recommended Belial in a couple of replies.
Experience
This is an image I created that represents how he most often appears to me, though he will sometimes change his appearance if he feels like it, and this definitely isn’t the “only” or “correct” way to see him!
△ Image created by Lady Eva in May 2015 & released under CC0 “public domain” licence
Belial seems to like you to show some backbone, I made a post here which describes that the first time he came into my life he attacked me, to see how I would react, and then (it seems) based his continued presence in my life on the fact I had to kick the butt of his army - a task I suspect he made achievable for me, although it wasn’t easy.
He’s shown a distinct preference to be evoked outdoors, there’s a small grove of trees nearby that has mist rising for a while after sunrise most days, and when I evoked him there one morning, he manifested some forty feet tall in that mist; when I subsequently summoned him indoors, he mimed that he found it cramped, but it’s something to be aware of.
This did somewhat backfire, insofar as it caused an outbreak of bizarre petty crime in the area – I posted about that in this topic last year. 
He doesn’t seem to be a stickler for more formal dress (King Paimon and Azazel, and Thoth, have all requested this, formal robes or at least fully clothed) - Belial doesn’t seem to give a hoot if I’m wearing a ratty old t-shirt and no knickers, and unwashed after a night’s sleep. He may be different with men, but I doubt it.
Sacrifice
He enjoys being offered things you wanted for yourself, such as half a tub of posh ice-cream I was saving, which makes me think the emotion of sacrifice is more important to him than superficial flashiness - kind of like the story about the widow’s mite.
△ Artist’s impression of this author, making tough decisions on demonic sacrifices!
I’ve offered him reddish-coloured things and he likes those, and he’s also said he’d very much like me to engrave his sigil in metal, aluminium in this case, so that might be a payment to offer him if he comes through with an important task.
Don’t offer it beforehand, he seems hard-headed and not the type to expect something for nothing. And don’t offer him anything unless you mean for it to become his.
He seems to dislike all things connected to empty status, he also seems to despise any kind of racial prejudice, snobbery, and income- or class-based prejudice, any effort we make (based on real or imagined criteria) to set ourselves above another human monkey such as ourselves. This has been my experience, and it’s also described in the Imperial Arts entry here, and that dude has a very different approach to me, and yet we received the same basic information.
This may be important, depending on your situation, and how to define the task you wish to achieve when working with him.
The Imperial Arts guy mentions in that blog that he got the impression Belial sneered at him, I’ve seen the same response but (for whatever his real form may be) the sneer or smirk he sometimes displays seem to me to be more the astonished amusement of a being totally outside human life, necessities, and desires, who hears the outrageous and ridiculous wishes of humans which must seem as surreal to him as the grimoiric descriptions of spirits who can make birds appear, or who ride camels and tigers, seem to us.
When Aaron Donaghue, who believes all spirits in the Goetia are aliens, attempted to remote view Belial, he was shown an image of a dead alien in a crashed spaceship - I believe that was Belial’s sense of humour showing through, to be honest! 
I did some work deconstructing his enn into Enochian, here, and his name, here - I can’t claim those are canonical and I’m not a language expert, but I think they offer a different slant on the traditional interpretation of his name.
I am extremely fond of Belial, as I posted here, but he’s informed me many times that he’s not a “friend,” and will only work with you if there’s something in it for him, so don’t try to plead to his sympathetic side (unless he personally tells you otherwise, he may be fond of some people in a different way) and keep things businesslike instead.
That’s about all I can think of about him, he’s approved the image I posted here and the sigil I posted here, and I’ve tried to capture the energy I get from him in both (the old “picture speaks a thousand words” thing…).
Good luck to all who want to work with the big guy!
