Demon is one my favorite roguelikes, it has been so for a while, but i didn't play much since i couldn't get very far, so i'd play a bit, lose horribly a couple of times and give up till i built up some will to pick it up again. (Demon feels harder, but more fair for both sides than my other favorite roguelike, DCSS, wich you seem to know very well )
Altho some of things that make me like this so much are things that you changed from classic roguelikes. It's like you picked up the roguelike genre, took out some rotten parts and added fresh stuff.
You removed equipment, wich makes people rely more on skill than on loot, while keeping the feeling of getting said loot by acquiring new demons, skills, and skills for demons. It keeps that sense of, awesome, i managed to get that cool sword, exept instead of it being found laying on the ground you convince it to join your side... I mean, there is a difference between going to an optional side place, destroying everything (maybe have a tougher time with a couple of enemies, or a straight up horde) and hoping some good stuff spawned in there for you, from going to Ccoa's sanctum and having to actually do stuff, rather than just surviving to get the big reward (Ccoa itself). If you don't get the big reward its not because you got unlucky, but because you messed up. Grabing a raiju is just a nice bonus that may or may not happen depending on your luck (and again, ability to conclude its link).
The enemies and abilites (most of them, Demon has a lot of skills after all, it's especially hard going by name, or abilities i havn't used yet, but that seems like a minor drawback of having a lot of skills) are unique and memorable. Maybe the demons are memorable because they are... Demons... Like, Artemis is Artemis, and a Raiju is a Raiju, but there may be more to that! For example, this happens with DCSS too: almost all enemies and spells there are recogniseable, and i don't think its just because i played it so much. I think anyone that has played it can instantly recall a Shock Serpent, an Orb of Fire, Grinder, a Toenail Golem, or even common stuff, like Adders or Ogre Magi.
Meanwhile in ToME or Angband, i can't really distinguish the enemies, most of them are just boring, they lack the Personality that Demon and DCSS have.
The combat is ability based, not just magic, everything is based on abilties, actives, passives, theres no boring regularly attacking enemies until they die. Altho not all of them are exactly the best (i'm looking at you, Fierce Stab! Don't laugh, Heavy Blow, you and your stronger variants that just cost more SP are the same! Just a stronger basic attack that you may not be able to infinetly use) they still have an element attached to them, wich makes them both better and worse than other similar basic stuff depending on the situation, the darts at least have a side effect according to their element, while i haven't seen a big difference between fierce stab and heavy blow as almost all passives that work on one work on the others (actually, now that i think of it, heavy handed or whatever only works with impact i guess). I guess them sharing that makes them differ from magic, its just that they don't between themselves. Light/Dark does similar things, but in more interesting ways.
The AI is cool! its not perfect, it can't be, and you're just a guy, but both enemies and allies alike don't mindlessly charge in, or attempt to hurt you with stuff you are immune to, or wait until its too late to heal their allies, or know your exact location so they can keep chasing you until they decide its not worth it anymore, or the reverse, or...
(sometimes they overdo stuff tho, you don't need to might the pyromancer when i'm fighting fotis, you have ice dart after all!)
The game is party based! I do love having allies, thats why i like JRPGs, it allows you to have specialized units instead of picking a class and going with it, and only it for the whole game, i'm not fond of concentrated power, i rather have allies to cover my weaknesses than just overpower them or negate them altogether (i mean, your allies are there to do what you can't i guess, but you still can't do it, no single unit can, exept for maybe heroes, but thats their whole thing, and they tend to lose... unless they are fighting me and they are called Madevra ). It also makes the game much less lonely, and even if you can't talk or interact them besides pimping them out, they're still there, they have a "mind" of their own, and they matter.
I also allways loved to play the summoner kind, ever since i played Dofus, about 10 years ago, i allways tried out summoners in any game they exist, hoping i'd like them just as much, but almost allways ended up being disapointed... But this game beat that standard
Progress: I don't know if it gets stale past what i've been to, but i doubt it...
It feels like i'm allways progressing and getting new stuff. Its not just about fighting new enemies, but changing myself a lot, not the summoner per se, but you're constantly getting new demons, fusing them, losing them because you're not good enough at this, and think they do not need to be dismissed because the enemy won't deal enough damage... Replacing them....
One time you have a healer thats weak to fire, then you have a different healer thats weak to body, then you lose your electric demon and can't replace it with a similar one, so you end up getting one focused on ice, etc...
Its much better than miraculously finding a Wyrmbane early on and using it until you finish the game (one way or another)
Emphasis on status effects tickles my fancy, there are negative conditions flying back and forth, lots of them, and all unique!
It's not a thing thats only viable early on, tough enemies aren't immune to them, they are not under/overpowered and they add a new dimension to the fights.
Those tend to either be neglected, useless against any midly strong opponent, or only dangerous to the player in other games.
In Demon they are a requirement i think, like dealing damage, or healing, but they are tacked onto nearly everything so its not that you need to focus on that (you can't really avoid using them anyway, even if you try), and as i said, they are like an extra dimension, after all, playing without ailments in Demon would be like playing almost every other game without dealing damage... so i really see no problem at all.
Talking about that, i also like the symetry, both the player and the enemies are in a relatively similar playing field. This works especially well with relic wraiths, as it really feels like PVP... in a single player game... a rogue-like nonetheless, and i love it, interacting with other players is cool, even if it feels like you're allways on the receiving end, despite getting informed of how many people your dead character killed.
(This is not a suggestion at all, just daydreaming... It would be awesome to have some sort of gamemode where you could draft or use your ingame characters and upload them / download other player's / or even use your own characters and pit them against each other... Since i'm daydreaming i can even pretend 2 AI summoners fighting each other wouldn't most likely be a stalemate. )
The last thing i want to say is that i love the theme of the game and how it incorporates real world mythology into it... i don't know much about mythologies, with most characters i know either being well known stuff like the Greek/Roman/Norse Pantheon and Kappas, Dryads, Sirens and all that stuff, or Demons from SMT (played quite a bit of that, me liking it is most likely related to me liking Demon as well, probably more of a correlation than a causation, bet its a big influence for you too!).
It's allways nice to see how they are depicted here, from reading the Codex to examining how their abilities and weaknesses/resistants relate to them, wich seems creative considering how generic those are (you give them thematic modifiers, but creating unique skills for someone would be overdone, so you gotta make use of what you have. For example, if you one day add powerful gods such as Baldr, you can't make a single mistletoe spear ability just to kill him, so i'd expect him to have extreme resistances but a weakness to Pierce in general, maybe even mind because of panic).
Same for their sprites too, you did a cool job, and by you i mean geminimax, because, assuming you did the original tileset, its worse than your UI game... much worse (i don't really have a problem with the UI tho, biggest problem is i can't figure out how to swap ability hotkeys, "=" doesn't seem to work... And the resolution not allowing me to see the bare bottom part of the game, the letters and numbers under the menu/ability icons, trivial... And me only realizing there was an options menu after watching a youtube video... And the Codex reseting every version, kinda hurts my little completionist side, not that i try to add all of them, but it feels bad having the progress reset like that... ). Roguelikes arn't exactly made as eye candy, but it gives the game a lot of flavour, and they are detailed enough to convey their looks while still living some room for myself to fill the gaps. Tell him i said "Bom Trabalho!"!
So yeah, just some words of encouragment, and telling you what i like about the game, counts as one player's opinion so you don't end up changing the game for the worse, i hope (all of what i said is intentional by you i supose, and are the core of the game, so i don't think you'd have a reason to change it anyway) Hopefully i will get better and end up beating it, after all, it only took me a few years to beat DCSS for the first time...
PS: The post is ridiculously big, and the punctuation is all over the place... It may be a bit hard to read but i was never good at writing anyway (i wouldn't have put such effort into a school project most of the time)