Here's a version of Summer Knight!Harry post-Changes where he got the mantle after Summer Knight. I figure the broad strokes mostly wouldn't change until Ghost Story: he'd still end up with Lasciel's shadow, he'd still find out that Thomas was his brother and help make Lara the power behind the throne, he'd still end up involved in Dead Beat (and still have to start working with Lasciel's shadow because Summer would be no help with necromancy), he'd still rescue Molly, still get involved in White Night), still end up rescuing Marcone and Ivy, Morgan would still come to him in Turn Coat, he'd still wipe out the Red Court in Changes.

Major differences would be:
-he wouldn't get made a warden in Dead Beat, and so would get involved with the war as Summer's representative (because the Red Court invades faerie territory in that book)
-as such, he wouldn't be training baby wardens, and would instead spend that time fighting on the front lines
-he would be able to react faster at the end of White Night, so he would still have access to the redeemed Lash
-the roles of Summer and Winter would be reversed in Small Favor, and it would be Summer raising a fuss about the Accords violation rather than the White Council
-as such, he still owes Mab two favors, and that's a lot bigger problem than it was in the books
-he'd do better against the skinwalker in Turn Coat, and Listens-to-Wind might or might not have had to step in
-Morgan would survive Turn Coat (Summer would help Harry with healing)
-he wouldn't kill himself after Changes

That's the point where things would go totally AU, so I'm not going to try to stat it out.

High Concept: A Dark and Stormy Knight
Trouble: Titania Hates My Guts
Other Aspects: Brothers and Sisters in Arms, I Hold With Those Who Favor Fire, Passion is Power, Protector of the Innocent, Stubborn as Hell
Skills:
Fantastic: Conviction, Discipline, Lore
Superb: Athletics, Intimidation, Presence
Great: Endurance, Alertness, Weapons
Good: Investigation, Deceit, Performance
Fair: Guns, Rapport, Empathy, Craftsmanship
Average: Might, Fists, Survival, Burglary
Stunts:
Arcane Lore (lore)
Infuriate (intimidation)
Too Fast to Hit (athletics)
You Don't Want Any of This (intimidation)
Powers:
Listening [-1]
The Sight [-1]
Soulgaze [-0]
Wizard's Constitution [-0]
Evocation [-3]
Thaumaturgy [-3]
Sponsored Magic (Summer; stackable) [-3]
Sponsored Magic (Demonreach) [-1]
Sponsored Magic (Soulfire) [-3]
Inhuman Mental Toughness [-2]
The Catch (threats to those he loves) [+0]
Refinement [-6]
Marked by Power [-1]
Magic:
Evocation (fire, earth, air, spirit, summer): +2 fire control, +1 summer control, +2 fire power, +1 air power
Thaumaturgy: +1 divination complexity, +1 biomancy complexity, +2 crafting frequency, +2 crafting control

I switched his spirit refinement for air because I thought it was more thematically suited, and I didn't give him strength/speed/recovery powers because per Paranet Papers I'm pretty sure he gets that from sponsored magic.

He also almost certainly has a bunch of refinements for item slots, and a bunch of really cool items, since per WoJ making items is one of his specialties and summer likes to give its Knights weapons rather than doing the Winter thing and treating them like weapons--I figure the Summer equivalent of Mab's "I'll try to kill you every day with no equipment" thing would be giving Harry a workshop full of magic supplies and some weapons training. (What kind of new equipment do you think that he would have?)

Think about what you wrote.  Both parties are warned about just this thing multiple times over the course of the story line.  Practically speaking, trying to apportion blame to them separately is  foolish. 

I have thought about what I wrote. From what I understand of what you've said, you are saying that Harry bears responsibility for what happened due to him having a daughter because he was careless about birth control. What I am saying is that even though he was careless, things would not have happened the way they did if Harry had known about his daughter. Therefore, more blame should be apportioned to the person who made the decision not to tell him than to the people being careless, because the carelessness was only a small part of the issue.

It's one of the theories I've heard proposed for why Lea's kiss burned Thomas: that when Lea bargained for Susan's memories, she also ended up with enough of Susan's love for Harry to burn WCVs.

She might also have wanted to know it what was between Harry and Susan was true love, and kissing Thomas after she'd absorbed Susan's feelings about Harry via her memories was a convenient way to test it.

...would the KotC try to break them out? I've heard a theory that the way to trap the Coins in Demonreach is to trap their bearers with them, thus subverting the "meant to be in circulation" issue--they are in circulation, it's just that their hosts can't do anything. The problem with this is that a coinbearer trapped in stasis on Demonreach couldn't repent (I don't think), so it seems like something the Knights would have a problem with.

I guess that could work. Honestly, though, I'm not a fan of the whole "you're overpowered until you get screwed over" paradigm.

Would it be better if it cost a point of sponsor debt? I don't want it to be too overpowered; I just want to make a power designed for characters who like to game the system*, and encourage character development away from doing that too much. A character with this power doesn't have to deal with a sponsor telling them what to do (ex. faerie queens), doesn't have to deal with dark powers like necromancy, and isn't tied to a location. Seems awesome, right? Until the character realizes that they're being driven to delude themselves, all their friends are upset with them because the character keeps lying to them, they can't have wards on their house, and on top of that if their house is attacked (or if they so much as trip and bump the wrong thing during a ritual) they risk fracturing their mind.

The power is supposed to have as many downsides as upsides--they're just not supposed to be obvious to characters when they start using it.

*By "game the system," I mean doing things like Constantine's trick of selling his soul to three demons, or trading your firstborn child to Summer only to turn around and make a deal with Maeve to get her handmaiden pregnant in exchange for something. Also, this power should probably not be taken by any character whose player is not willing to have the mirror destroyed in gameplay.

As I recall Lasciel fights Harry in Skin Game. Can't remember what form she takes, but I'm sure if you dig that book up and look at the end it'll have something useful for you.

She just makes Hannah naked and covered in a few wisps of purple mist. Didn't really seem to suit Harry.

Do you have any aspect suggestions? I want to include something about his friends and family, but Jim has said that Lasciel would work to isolate him, so I'm not sure what to say about them.

Also, as an alternative to allowing you to specify a second target for thaumaturgy done with the mirror, do you think it would work for it to let you gain shifts equal to your conviction without having to roll to control it (sort of doubling your innate power, rather than the power of the entire ritual)?