Firstly, I'd like to say I /LOVE/ this class. The flavor, the mechanics, the way it approaches the world is fantastic. I love it, and want to love it. I'm sad however that as-stands, it's objectively a weak class. I read through only the first couple of pages of comments, so apologies if I say anything redundant. I just /really/ want to see this class be fantastic.
My opinions and suggestions:
1. As has been mentioned numerous times in the first couple of pages alone, the 3/4ths BAB on this class really hurts. In combat, none of the currently revealed Spirits grant powers that can be used as attacks, meaning you're using actual, mundane combat (Though maybe with bear claws?). If you choose to use one of your precious few bound Spirit slots (Primary-Quaternary) on a Strength spirit, then you can rise to the level of "Not godawful" where it really ought to be sufficient to get you to "Actual a decent melee combatant". What exactly is a Medium intended to be doing every round? Without abilities that offer alternate means of attack, they will be fighting with a mace or spear, very crappily, with sub-par combat stats and health. Either up their HD to D10 and BAB to full, or give them other means of compensating. Personally, I think 3/4ths BAB fits flavor-wise for the class - You've spent your days talking to spirits, not training with a sword or bow. The simplest fix I think is either increasing the benefit of the stat bonus from Strength spirits, or allowing a sort of Trance-Rage type of deal.
2. I ADORE the Twins. Absolute favorite, hands-down. If it was possible to make it useful in earlier levels though, that'd be great. As-is it's pointless to take it before mid-level.
3. You should be able to swap out old Spirits every four levels or such as per a Sorcerer. No need to lock players into their early choices.
4. Spirit bonuses in general seem low for abilities that reference them, and for the Strength stat bonus. Getting a +1 to a skill is REALLY lackluster. I'd want a +4 at least if I'm spending precious character resources to get it. Perhaps have such skill bonuses be 2x the Spirit bonus instead? Or just consider having the Spirit Bonuses go up faster.
5. Trance shouldn't have the per-day limit as written. You already have the limiting factor of having to keep Influence down to prevent players from spamming it. Trancing is your #1 source of versatility, which SHOULD be what this class is all about. I see no reason to make it at-will. Out of combat, a Medium ought to be fulfilling the role of a utility caster, doing odd-jobs with Lesser Powers. If they're confined to only once or twice per day, they can't do that very effectively.
6. Consider expanding on the Influence system. As has been mentioned, GMs controlling characters is a very iffy thing. Perhaps something like a Geas effect, where the Medium gets increasing penalties as long as they refuse to take actions that oppose the Spirit influencing them? Especially if you remove the per-day restriction on Trance, I'd recommend you consider a modified system of dealing with Influence to increase player interaction with that part of the system (Which currently does and means very little). What if Influence was more granular, and had effects at earlier steps? What if Influence's break-point was determined in part or wholly by your Charisma modifier? What if you had a means to somehow control which spirits' influence go down by how much? Maybe you can voluntarily take on more Influence when you Trance in order to get the Intermediate power from the spirit you're trancing with?
7. Fool's Lantern stands out as being the only current Spirit that directly deals damage (Respectable damage, at that). Sadly, there seems to be a pretty significant problem with it - Enemies can just choose to NOT ATTACK IT. Right now enemies have zero incentive to attack your flying bomb. It can't distract them, it can't flank, it can't deal damage, it can't grapple, and you can't tell it to explode. Why would an enemy ever attack it? And in shield form, the 20% miss chance is pathetic, given that you need to spend a Standard Action to make a wisp, and the wisps are one-time-uses. At least Mirror Image gives you a 50% miss chance for that one attack. Easiest solutions would be to boost miss chance to 50%, and add something that allows you to either force-explode your wisps or add something so the wisps are annoying enough to enemies that the enemies would willingly choose to take the damage.
8. The Lost is a fascinating and flavorful Spirit. I love the idea of voluntarily going insane to wail on opponents. However, as-is, it's very meh. You're increase in control isn't sufficient to allow you to actual assure with ANY reliability you won't be hurting allies, and it's essentially a needless cost in exchange for a bonus you could get without risk from a Strength spirit, in addition to that spirit's powers. IMO, when you go insane you ought to get a much more substantial boost to combat abilities to balance out the risk, and the Spirit Bonus ought to be a lot more meaningful, as mentioned previously. On normal skills at least +1 means a 5% increase. Here's it's only 1%. I'd recommend Spirit Bonus x5 or more.
9. I'm confused by the spellcasting progression. It looks like it was intended to be the same progression as Rangers and Paladins, but is just a little off from that for no apparent reason.
10. Read Realms of Chaos's Xenotheurgy system. It's similar in a number of respects (Drawing power from a dangerous and unpredictable source) and may give you some ideas - http://www.giantitp.com/forums/showthread.php?122103-Xenotheurgy-Far-Realms -magic-system
11. The majority of the Seance Boons seem either inconsequential or extremely circumstantial (And not even great in those circumstances). Why would I want to give my entire party +2 to Sleight of Hand? There's likely to be at most one or two such checks made in any given day unless you're in a campaign all around such checks. As a comparison point, the boon from The Beating actually feels useful and meaningful while still being flavorful. +2 to damage on hits after the first? Hells to the yes. I'll take that over a +2 to a random skill any day. Skill bonuses are fine, but they aren't going to be helpful to the entire party all day, and this is an ability that affects the entire party all day.
