Trying to incorporate Espers/summons/GF's from Final Fantasy

[KHillBox]

I recently started playing the mobile game Final Fantasy brave exvios and it has Espers in it your characters can "equip". The Espers can level up and you can unlock abilities for the character they're equipped to. I like this idea, so I was thinking I'd like to try and find a way to incorporate something like this into my next campaign.
What I have so far is when you encounter the Esper(typically sealed somewhere so it can't run around) it puts the PC's through a test(combat,puzzle,riddle or just wants money depending on which esper it is) and if/when they pass the test they can accept the espers power and the esper fuses with the PC's, not sure exactly what to call that, fusing, merging, joining, something like that.
Anytime XP is gained by the character that has an esper "equiped" 10% of it goes to the esper, say you get 700XP from a fight they have to allocate 70 to the esper and the character gets 630.
I was thinking they can only have one esper "equiped" at a time, maybe add a feat that lets you have two?
The relationship between host and esper would be symbiotic, the character could hear the esper speak to them but no one else can (thus letting me give them the "don't touch that dummy" or "go there" directions without it being me :P) and the esper would retain knowledge of previous hosts they had time with and was thinking the XP they syphon would be what sustains their immortality.
Not sure how I want the separation to go (when an esper leaves a host), if the host says to leave they have to, if the esper just up and chooses to leave, if the host is killed, not sure how I want that to go and any adverse effects, open to ideas there. I would like it to not be a permanent thing though.
As far as the espers leveling up, I was thinking the amount required to level would be 10% of what it is for the PC's plus 40%, so to get to level 2 is 1000XP for players, for an esper it would be 140XP so they level up slower than the PC's
As for powers/abilities, I was thinking each would be different, some might have a couple of the same abilities, but each would start at lvl 1 with the ability to be summoned once a day, like Shiva would do ice damage in an area that would increase with her lvl. As the esper levels up it would unlock more abilites, say something like the esper Titan gives the toughness feat line as he levels up and maybe a bonus to AC things like that.
Not sure if I want them to have a level cap to start and then have some way to unlock them further.

So I'm really just looking for advice/criticism here, what do you think? would it be good/fun? I'm not worried about them being overpowered necessarily, I like the hero campaign idea, and plus I can laways just give the BBEG a really strong one to even it out lol

[DrMotives]

This sounds overly complex & your players will see "it's costing me XP". You could refluff a couple of existing me mechanics to do this though. One is using the bloodlines from UA, or look at the Weapons of Legacy to create a unique "esper" slot instead. While I've not looked too much at the Legacy rules, if you went with refluffed UA bloodlines I'd start with your acquisition method to bind an esper in the first place, then have it unlock new powers at the PCs character level. Possibly with other required quests at other points to let it continue advancing, but I can really see people not wanting to pay XP into a spirit cohabiting their body.

[Extra Anchovies]

The problem with XP costs is that they don't work well unless you and the players are fine with accepting a level disparity in the party, and even then they don't work well. The reason for that is because they only actually have any effect when the player is between [XP needed for next level] and [XP needed for next level, minus total XP costs paid this level], but when it does have an effect it's a big one (you're one level less than everyone else).

Instead, ditch XP costs altogether, and give an esper to each member of the party. If that wouldn't make much sense in-game, give an esper to one of the PCs, have the party figure out what they are in-setting, and have the next quest or two be finding espers for the rest of the party.

I'd recommend the UA bloodlines, but those are restricted to creatures which can breed, which is boring. Why pass up the chance to have, like, Aboleth espers? Having an antediluvian horror bound to your soul would be awesome awful awesomeful! Instead, how about having an esper provide the Bloodline Powers from the various PF Sorcerer Bloodlines (http://www.d20pfsrd.com/classes/core-classes/sorcerer/bloodlines)? If you want to make them more powerful, you could even give each of the bloodline spells as a 1/day SLA.

[daremetoidareyo]

Tome of magic has vestiges, which is probably a better mechanic. Just steal it. Call them espers. And make your binder level your character level.

[Fouredged Sword]

Maybe do a gestalt with a very restricted 1-6th level spellcasting progression you can exchange around by equiping espers with a 1/day ability where you summon it for a great effect once.

For example
Bard progression spells
no bab, skills, saves, or anything else
Spells count as divine and treat CL = character level. Cha is casting stat.

Spells know driven by esper with a 1/day summon effect, so for example

ifrit
- spells are mostly fire damage AOE's like burning hands. Say 3 per level except only 2 5th and 1 6th level spell.

Summon - 1/day you summon Ifrit to impose fire on an area in battle. This takes the form of ifrit floating surounded by 4 spinning pillars of fire. This occupies a 40ft diameter cylinder 100ft tall. All creatures the user wishes Ifrit to attack within the area suffer 2d6+cl fire damage. On a successful reflex save this damage is reduced to half as the target dodges the spinning pillar. This effect lasts for CL rounds and will move 10ft per round in any direction as the caster wishes.

[KHillBox]

Thanks for the ideas!
I'll have to check out the UA and vestiges to see if there's somehting that will fit from there that I'm looking for.

I figured the XP penalty wouldn't be that bad if you're getting abilities to cast some spells, getting feats and ability bonuses.
Ya I was thinking they'd find one at first and have to choose who gets it and then I could use that esper communicating to the player where nearby power is (other espers) for them to find more for the party so no one gets too far ahead and I mean even if everyone is at level 20 and someone had -10% XP from lvl 1 they'd still only be a level behind, that's typical for our campaigns, when people don't show up they miss out on the XP that session so I don't think it's that bad.

But I'll still check those books out and see what they got.

[redzimmer]

Tome of magic has vestiges, which is probably a better mechanic. Just steal it. Call them espers. And make your binder level your character level.

And if you level up with an "esper" bound, you get permanent #-use-per-day SLAs.

[daremetoidareyo]

And if you level up with an "esper" bound, you get permanent #-use-per-day SLAs.

This is a great addition

[Prime32]

IIRC, in Final Fantasy 6 equipping an esper would let you summon it (which acts as a flashy but inefficient blasting spell), and levelling up while the esper was equipped would teach you spells permanently.
Basically, it's the exact same thing as D&D's multiclassing mechanic.

So in D&D terminology, each esper is an artifact with an associated prestige class. If you want players to master multiple espers, then make them short prestige classes - 3 levels each.

[Telonius]

Thinking about the whole Esper mechanic, the closest thing I can think of would actually be Incarnum. Espers can be switched out when you need them (check), grant various effects magic-like effects (check), can become more powerful if you use them long enough (compare to chakra binds). If I had to incorporate Espers, I'd probably just make them each shape a Soulmeld, and grant a specific Chakra bind by sacrificing a set amount of XP.

[KHillBox]

Lots to work with and lots of good ideas/changes. Thanks people! I think I have enough to go on here :D