Since no one has come up with an actual permanent ally god proposal yet, here's my (off the wall) attempt at one.
Hebe, the would-be god.
Flavor: Hebe is a Demigod with potentially great powers. These great powers are undeveloped though, and only through the power of the Orb of Zot can Hebe gain full access to her* divine ancestry and abilities. Hebe can be found near her altar, a simple, home-made affair that only occasionally pops up in the dungeon. Upon encountering a potential worshipper, Hebe offers a bargain. In exchange for worship, the adventurer gets Hebe's services as an ally. Of course, demigod adventurers are treated as competitors and would not receive this offer.
* Yes, crawl gods don't have gender, but Hebe isn't a god yet
Hebe is an ally with a randomly selected background (based on the backgrounds available to players). This background determines the choice of gear that Hebe will prefer. Hebe will have access to your stash and will simply pick the best available items according to the background preference. Hebe may gain some spells (if any) as manifestation of godly abilities. In addition, Hebe can learn (some) spells from your stash/inventory, again depending on background. An active command (Apply Consumable) becomes available through the god screen; the target could either be a potion or scroll from inventory that Hebe will then use.
Piety: As Hebe is in no position to demand extravagant conducts, piety simply accumulates over time. Piety gain is higher the deeper the adventurer goes into the dungeon.
God Abilities: Not much to speak of, but at 5*, the player gets the ability to banish Hebe to her home plane. This however uses up half of the piety built up with Hebe. The player then gets the ability to recall Hebe, but this uses up all remaining piety. This process will completely heal Hebe, however.
Final god gift: Upon obtaining the Orb of Zot, Hebe becomes a god and uses divine powers to wipe away all enemies on screen. Hebe will then be busy with new god duties, leaving you to do the orb run alone.
God Wrath: If the player abandons Hebe or lets Hebe die, the player will start at negative piety with any new god. This is because all the gods of the crawl pantheon (including the outcast Lugonu) consider Hebe to be of divine blood, and don't like the shabby way you've treated her.
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It's something of a fantasy adventure trope I guess, but I hope some of you like it!
Generally, I expect Hebe to be weaker than the player, so keeping her alive would be part of the game. I would say that it's not a particularly strong god, even with the very good tactical final gift. Definitely a niche sort of god, so I wouldn't expect Hebe's altar to appear every game.