Spoiler: ECL 5
Show
We start our adventuring career with a level of ranger. Every class feature gets swapped out: Rival Organization is chosen over Favored Enemy because of the skills it boosts and its more defensive nature (boosting Gather Info, Knowledge, Sense Motive, and AC), and Skilled City-Dweller gives us four skills we all need and is thus a no-brainer. Urban Tracking will go great with the Gather Information reqs that the SI demands, and Spiritual Connection is just better than Wild Empathy for ranger dips.
Our cleric level is sadly mandated for Shadowspy entry, preventing us from picking a more interesting divine class. We grab healing devotion because we only have around 15 HP, and even with DR it's nice to have a built-in source of healing that'll kick in whenever we go down. True daylight will help against undead, which are going to be a bit of a problem for our default strategy; we're willing to give up the Strength domain's granted power for it.
What that strategy is? Well, grigs can generate an Irresistible Dance effect by playing their fiddle. I'm choosing to interpret this as a standard action to initiate and then a standard action to keep going each turn, but even so it's incredibly good, forcing save-or-loses on a huge area at-will. It makes up for our awkward LA and basically guarantees we'll be useful in most battles. If we can affect at least a single enemy during each round, all of our melee allies get a free attack with boosted to-hit, while that enemy skips its turn.
The ability is mind-affecting, but mind-affecting immunity tends to be on creatures that lose to our other main draws; flight and stealth. Vermin won't have the skill points to spot us, constructs only rarely fly, oozes fail on both counts, plants rarely fly and those with skill points tend to be on friendly terms with fey anyway. That leaves undead, against which we can currently attempt some weak Turning, cast Hide From Undead, or simply buff our allies with spontaneous Enlarge Person. We also have Entangle as a 3/day SLA with a respectable DC of 15, which corporeal undead get screwed by, so the things that mess up our primary strategy are by no means stonewalling us.
Out of combat, we help out by being extremely hard to spot (+17 hide before items or magic, and 3/day Invisibility when needed), talking to plants and animals, and employing Urban Tracking. We also have a respectable modifier to Sense Motive and various social skills, making us a bit of a face early on, and our +3 to Lore will retrieve some useful knowledge from time to time.
Spoiler: ECL 10
Show
Radiance of Pelor is so mediocre that even I can't give a pitch for it beyond 'we have low-light vision so we benefit extra compared to humans or something'. Aura of Anonymity is good because free skill bonuses are always good, and having flight lets us enter or leave crowds easier than most.
Immunity to Blindness, however, is amazing for us. Grigs have 3/day Pyrotechnics, which is generally too hard to use to be worth it. Either you rely on external fire, which is hard to target and unreliable, or you carry a fire source with you and include yourself in your spell's AoE: neither is appealing. But with immunity to blindness, we can carry a tindertwig, turn it into a flashbang, and be no worse for the wear! (obviously never use the smoke variant on stuff you're carrying)
Also: thanks to Vow of Nonviolence our Entangles and Pyrotechnics SLAs, fiddleplay, and most of our spells just got that much harder to resist for humanoids and monstrous humanoids. The DC to avoid irresistible dance, for such creatures, is 23, and it's set to increase to 25 in a single level.
Personal Eclipse boosts our hide even further. Veil of Pelor seems redundant with our new 2nd-level spells, but it's an immediate action, and can thus be used in response to divination spells that seek out magic auras. Arcane Sight is an underrated means of locating enemy sneaks, and you can at the very least create irrelevant auras for it to pick up on. Ventriloquism works for this as well, and will make the enemy think twice about assuming that any illusion aura is your invisibility spell. Unlike other PCs, we aren't very reliant on weapons and armor, so we might even be able to get away with shedding our magic gear entirely for brief missions, leaving only the fake auras.
Anyway, on the casting side we get Close Wounds, Silence, Detect Thoughts, Lesser Restoration, and the busted Guidance of the Avatar (or Divine Insight, if your DM bans GotA). We mostly use our cleric casting for buffs and utility, but our Vow helps keep the few spells with saves competitive. Find Traps and a skill-boost spell combine for an 1/day burst of trapfinding (helped by the grig's racial Search bonus), but we don't have to worry too much about those while scouting; traps can fail to show up on Detect Magic, or target flying tiny-sized creatures, but rarely both.
Spoiler: ECL 15
Show
Thanks to our shadowspy bonus feat, we enter vigilante and begin to advance it with shadowspy. The vigilante list is surprisingly good, consisting of the bard list minus its conjurations (eh), evocations, and enchantments, and thus still including the many great transmutations and illusions on there.
(By the way, while vigilantes only get 4th-level spells, their list is a subset of the bard list and includes higher-level spells because of how it's defined. The build does not rely on it, but assigning a portion of your WBL to scrolls and wands of 5th-/6th-level bard spells is totally doable).
Catsfeet and Accelerated Movement both help out with sneaking, Pyrotechnics is now a spell in addition to a SLA, letting us use it even more (again note that intelligent mind-affecting immune foes are, by and large, not immune to reflex-save blindness). Alter Self is a bit limited because of our tiny size, but we can still turn into rime sprites (tiny, 80 ft. perfect flight, 60 ft. swim, str/dex boosts) and bog imps (small, 60 ft. burrow, str/dex boosts, +8 hide bonus in swamps). Notably, once we turn into a bog imp or other small-sized creature, we can then use our Disguise Self SLA to assume the likeness of a halfling or gnome, finally letting us use the disguise bonus granted by Shadowspy and helping us disappear in the crowd when needed. This might give a +20 modifier on your final disguise check, but that's a kind of silly bit of RAW so ask your DM (at any rate you should get +10, and increasing that slightly for using two spells seems reasonable).
The vigilante's streetwise ability also boosts our Urban Tracking checks, which is nice, and Detect Evil at-will helps us identify which people need our moral guidance.
Vow of Peace sends our AC into the stratosphere, shatters weapons that hit us, and calms all creatures nearby. Ask your DM whether playing the fiddle counts as an 'aggressive action'; if it doesn't, you can play until the calmed creatures start to dance, then act freely while they remain incapacitated. Between its bonus to Diplomacy, Sacred Vow's bonus to diplomacy, a massive charisma stat and a synergy bonus from Sense Motive, we're looking at +16, and a quick Friendly Face bumps that to +21; the barest expenditure of resources makes you a face on par with the 16 charisma max-ranks paladin! Stealth and diplomacy help minimize the annoying effects of your vows: you don't need to worry about taking prisoners if you evaded or befriended them!
Our return to shadowspy yields us Stealthy and Extra Turning as bonus feats, which help fuel our stealth and divine feats respectively. Note the comprehensive defense we're setting up here: spells run into our high saves, melee weapons will shatter upon striking us, and ranged weapons need to eat a flat miss chance thanks to Air Devotion (Pelor is no Air god, but the sun is in the sky, sunlight causes wind, and we literally fly; there was enough thematic synergy there to me and it's RAW-legal anyway). Unlike the typical tanky character, enemies can't simply ignore us, because we're still spreading nasty debuffs with each action.
Truth of the Light and Sun's Revelation both get the +4 to their DC from Vow of Nonviolence, which helps us now we're no longer pumping Sense Motive. Our giant charisma modifier is useful here as well; shadowspy pretends to be a wisdom-based class but has way more features that care about charisma. Although we can prepare Zone of Truth in our cleric slots, it will have a much lower DC: this SLA is not redundant! The fact that we can cast it without somatic components and as a swift action means we won't have to stop fiddling to get it off, letting us smoothly transition from combat to interrogation.
Blinding Light is great for this build. Normally, the issue with a 2/day save-or-suck is that you're going to waste some of the uses on unfortunate rolls, so there might be days you won't even get use out of it. However, our Irresistible Dance effect imposes a -10 penalty on Reflex saves, which makes Blinding Light great for screwing over everyone currently dancing when we need to go and do something else, and because it's only a swift action we get a full round of actions afterwards (and, in the unlikely situation our targets made the save, we can just keep playing). Instead of gambling with the limited uses of Blinding Light, we can gamble with an at-will ability and only use the flashbang when it's guaranteed to work.
Spoiler: ECL 20
Show
Mystic Theurge gets used to bring us to 4th-level cleric spells and 4th-level bard spells, and also gets us the Spellcraft ranks we need to more reliably identify detection spells being cast. Glibness can end social encounters all by itself, Remove Curse is great utility and always good to have on your backup caster, Shadow Conjuration is ridiculously versatile, and Haste/FoM are both insanely good buffs.
Persistent Refusal gives yet another feat to channel our Turn Undead uses into (we're up to 15, so this is not a scarce commodity), and lets us make new saves against ongoing spell effects, which is most of the really bad ones that don't instakill you. And as for those instakills, our cleric levels do give us Death Ward... Really, Jeremy represents a build type I really like, where your to-go trick is so capable out of the box that you can focus entirely on maximizing defenses.
We also grab Hide in Plain Sight, letting our tiny self with near-maxed Hide ranks disappear entirely. By the way, ask about playing a fiddle while hidden; though a DM might argue it breaks stealth by making people 'observe' you, you can now hide in plain sight anyway! Note that up until now, we probably had to expend resources to get into fiddle range versus faraway foes; but wide open spaces tend to be sunlit, meaning we can just sneak through them! Furthermore, ask your DM if our true daylight ability renders areas sufficiently 'sunlit' for the purpose of HiPS.
True Seeing is one of the nicer cleric spells that we miss out on, so getting it as a SLA is great. The fact that it's lacks a verbal component also helps with casting it while stealthed, though that's an upside I imagine most shadowspies get out of it.
For those who've been keeping track, our fiddle now forces a DC 26 save before items (30 vs humanoids and monstrous humanoids), our Gather Information is up to +32 and can easily be bumped further with Friendly Face or [divine skill-boosting spell of choice], and our SLAs have a DC around 20. Our AC is 24 before any items and can be very easily increased, our touch AC is high enough that low-BAB wizards will still struggle to hit us, and we force enemies to make multiple saves before their weapon even breaks our skin. We can target both Will and Reflex to deliver potent debuffs at a surprisingly high DC, covering most of the monster manual and ensuring our allies will always appreciate having us around (worst case, we spend a combat applying Hastes and Enlarge Persons). Out of combat, we get as much out of vigilante as we can and get access to whatever cleric spells are most useful at the time; we're no full caster, but we can easily relieve some pressure from a party whose main arcane/divine caster has limited spells known.