I was reading "The Wrong Way to Use Healing Magic," which...OK. I thought it was just a generic isekai an interesting title at first, but it's an interesting enough read that I am enjoying it, and decided I will make a character build for the main character (obviously, a healer...who uses his healing to just keep fighting and training ridiculously...forever).

Race: Dwarf
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This is for Con and Wis. They are hard headed, both literally and metaphorically. Since we will be casting from Con, this is important.
The Stubborn Alt Racial trait is more useful than poison resistance. You can't heal yourself if you're mind controlled. But you can cure poisons. Therefore...
Oh. And they also aren't slowed when heavily encumbered. So long as your packs can be quickly set down before combat, you can carry a bunch, baseline.


Ability points [20 PB, post racial]: 12, 12, 20, 10, 12, 7
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After constitution, we don't *really* need anything. Also represents the character pretty well. He's shown to be only slightly more muscular than an average protagonist, but has stupid stamina. Also, every word that comes out of his mouth...anyway, 7 charisma works.
Put all level up points in to constitution.


Classes: Mageknight 2 / Incanter ? / Striker ? / Something else
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Sage 10 would actually be the most appropriate for this character, thematically and all. But it's cheese as buck.
Mageknight is for Imbued Weapon, which lets us spend a spell point to use our Con mod for attack rolls for Con mod rounds.
Incanter is for talents.
Striker gives Constitution as practitioner modifier, and is the only way to get it as such. Would recommend first level.
Probably sneak in a level of Bloodrager or Barbarian or something.


Casting Tradition:
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GENERAL DRAWBACKS
Draining casting: Take 3 points of nonlethal damage each time you use a sphere ability (at level 10).
Emotional Casting: Unable to use magic when subject to non-harmless emotion magic.
Magical Signs: Glow a deep green whenever you cast.

Fey-Infused Magic: Less effective to iron. (Manga talks about the energy flow of mana. Reminiscent of nature spirit stuff. Optional.)
Painful Magic: Fort save or be sickened on cast. (Optional. Doesn't fit the character. But con-focused caster. Easy mechanical fit.)
Strenuous: Max 1 SP per round. (Optional. You don't have any abilities that need more than 1 SP per round.)

LIFE SPHERE DRAWBACKS.
(All of these are early game drawbacks. Buy them off when you can.)
Glorious. Gives Taste for Victory (a talent you want), but can only use healing when ToV is activated.
Regenerate: Only target self.

ENHANCEMENT SPHERE DRAWBACKS
(None of the following reflect on the character. It's pure mechanics. You don't even need to get half of them to get the most relevant enhancements for 1 talent.)
Personal Magics: Only affect you and your equipment
Narcotic Magic: -2 to will saves while affected
Enhancement Dependency: -2 to Fort saves while not affected
Delinquent Enhancements: Can't concentrate on them
Bodily Enhancements: Only affects creatures.
Consciousness Linked: all enhancements shut off if you aren't conscious.

BOONS
Fortified Casting: Use Constitution as casting ability modifier.


Sphere Choices

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LIFE SPHERE
(Of note: the various Restore X talents are less effective, on average, than Greater Healing at level 5 onward.)
Base sphere, complete with Invigorate, Cure, and Restore.
Greater Healing: +CL hp per cure. Stackable 4 times.
Greater Restore: Cures rather than lessens conditions/ability damage (particularly exhausted, for character purposes)
Restore X: +1d8 healing, and cures certain ailments.
Taste of Victory: Hitting a creature grants swift action healing. (Should also take Ranged Healing with this.)
Fount of Life: Store 10xCL hit points, and dish out as you please.
Resuscitate: Cure the dead.
Self-Renewal: Cure/Restore self, even if otherwise incapable, due to a condition you can Restore. Even cure if unconscious. Near immortality.

Disruption: 1d12 dmg / lvl per SP, when maxed out. That's substantial. But it lacks the AoE and CC potential of Destruction sphere. Still, you'll be the envy of most any non-uber charger Barbarian and their axe.
Affliction: 2d6 rounds of Exhaust/Fatigue base line is impressive. Also other possible disables if you took the Restore X's.

Aggressive Vitality: Useful for ToV, also any martial character.
Sustaining Vitality: Holy cow the defenses.

ENHANCEMENT SPHERE
Base sphere: Improved atk, dmg, AC and Shield bonus.
Deep Enhancement: lasts 10 min / CL now.
Natural enhancement: Allows for bonus to natural armor. Meh.
Improved Strength: If really needed. Also allows wielding of larger weapons.
Physical Enhancement: You ought to have magic item for Con, but here.

ALTERATION SPHERE
Base sphere: Change cosmetics/add natural weapons. Meh.
Agile Transformation: Initiative +4, Dodge +2.
Greater Transformation: +1 trait. Good early on. Retrain later.
Lingering Transformation: double duration of shapeshift.
Powerful Limbs: Larger weapon size, move speed, and carry capacity.
Size Change: Actual larger (or smaller) sizes.

BERSERKER SPHERE
Free action Toughness as temporary hit points each round for -2 AC.

GUARDIAN SPHERE
3x BAB as delayed damage. When you can heal it as a swift action. Just all that much more of a buffer before you need to commit to healing.

Literally every talent here is good in some respect. You are a heal tank. Being able to direct that attention to you is good. Patrol is arguably the weakest package, but it can proc Taste of Victory.


Feats:
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Extra Magic (or Combat) Talent: Bonus talents. Bonus yay.
Extra Spell Points. 2 points per feat.
Wellspring of Life: Double Fount of Life's capacity with max Greater Healing. That's up to 200 HP of storage at level 10, accessible as a swift action.
Extended Resuscitate: Yay. More time dead then revived.
Advanced Magical Training: treats non-caster classes as 1/2 casters for CL.

Improved Initiative: Not so needed, as healing is innately reactive.
Favored Form: Base form, add more traits. Good early on. Retrain later.
Combat Casting: Worth 4 character levels worth of Concentration DC, if you're mixing it up in melee, like the character.
Vital Strike: If you're going in on the Big, Single attacks, and have used the various spheres to boost your weapon size, go ahead.
Robustness: +HD, whenever you heal, up to double. With Fount of Life, you can ensure you have this efficiency going on. Or you could spend the feat on the Revitalize Talent, if you want slow efficiency.

Power Attack: Some people swear by it. I am not some people. I prefer dealing moderate damage every turn over dealing massive amounts every so often, twiddling my thumbs until I get a good roll. Also, this really isn't the character's role.


Traits
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Gift for Magic (Magic): +2 CL (to a max of HD) to any one sphere.
Mastered Talent (Combat): +1 eBAB for talent, +1/4 levels after 1. In case you need more Guardian delayed damage.
Desperate Focus (Magic): +2 Concentration. Decent, if you're in melee. Prob not worth
Healing Reserve (Magic): +1 SP for Life sphere, for 1 HP. Worth the HP. Maybe not the trait.
Natural Shifter (Racial) +2 CL (max = HD) to Alteration.

And then all of the other naturally good ones, like Perception as a class skill, or bonus initiative, and what not.


Magic Items:
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The normal main stay items (ie. your stat boosters, deflection, Natural armor, miss chance, and Resistance, and so on)

Gloves of Elvenkind (7,500): +5 spellcraft and concentration
Sphere Staves: Adds Enhancement modifier as CL to select Spheres
Shield of Close Calls (Spheres of Power armor): Heals you on death, potentially saving you, letting you heal the rest of the way.