First off this is a going to be a fairly large post with my various ideas...sorry about that but the first few paragraphs ARE the most important.

OK to start of I'll say that this is gonna be alot of flavour based stuff but I don't want to completely drop the concept of optimization..IOW flavour isn't fun if your dead or a waste to the party. So say about 50/50 focus.Note: I'm not against stretching or "fanficing" a few things in favor of optimization or just coolness.

I started this build out as a normal summoner and stumbled onto synthesist which sounded cool but I was going to skip past it until I had a picture of Alphonse show up during a search for "Armored Knights" (I was going to make a quadreped + 2 legs + 2 arms + x nat armor+ martial weapon Lance to reflect a knight riding a dragon) and just got this wonderfulyl crazy idea where ALphonse would be my summon and I would make Edward the "summoner" but than I thought it would be really cool if Alphonse and Edward were to merge somehow so I jumped back to synthesist.

Anyways that explained let's get down to business I'd like to cram as much FMA jokes/lore into this as possible. (Note: I'd like to focus on the earlier half of the Anime where its fairly light-hearted with lots of fun jokes,etc. not the dark turn in the latter half)

Oh and I'm open to multi-classing but preferably limited to light dips and only if it adds something to the flavour or to optimization.

Ok This is what i have so far...I can't finish it cause I have to go to work but feel free to make suggestions in the mean time...I'm going to try to be very open for this build.

FMA - Synthesist Summoner Alphonse+ Edward Build

Race: Quarterling (custom race abilities focus on small size and boosting Cha and some Int...making fun of Edwards Small Size/Napoleon Complex..also an exuse to finally make my "Quarterling" race)

Eidolon: Alphonse, heavy armor look focus on Str/Con, make as large as possible, mostly Slam attacks possibly breath weapon because its funny

Summoner Symbol: The Flamel

Stats: Cha -> Int -> Wis

Possible Skills: UMD, Spellcraft, Linguistics,Intimidate,Bluff, Appraise, Stealth,Diplomacy, various knowledges (using traits to add class skills)
Traits: ???

Possible Feats: SF: Conj, SF: UMD, SF: Stealth, Augment Summoning, Toughness, Improved Ini, Expanded Arcana, Additional Traits, Combat Reflexes, Improved Natural Attack, IC: Natural, Eldritch Claws, Lunge, Toughness, Resilient Eidolon

Spells: Focus on buffs mostly

Oh should I be taking the TWF line of feats? since I'll be doing multiple attacks...actually how does that work I'm really confused how the multi attacks thing works.

Oracle 1 -> Nature Mystery -> Whispers of the Land: Cha to AC instead of Dex. I don't think the class would suit Edward or Alphonse though.

Oh one thing I'm thinking about it having this be much later than than the show and have Edward be really old to the point where he pretty much needs to be fused to Alphonse to survive. (remember this Al & Ed never made it to the "dark" part of the anime)

Also an interesting feat is Dimensional assault which uses dimension door (synthesist as an SLA at lvl 6 1/day +1/3 per summoner level) to allow you to "charge" with a teleport.

Arcane Savant & Boon Companion
Two levels of AS makes UMD, Spellcraft & Knowledge Arcaner 10x more awesome, grants me extra spells...either more Summoner spells or ANY SPELL (one level higher) the possibilities of those 2 things alone are epic and for pre-reqs I'll have anyways (except the item creation feat but 1 feat can't hurt) than with boon companion I have room for another dip class or two. Thinking Synth 16 [+ Boon companion for Lvl 20 eidolon] Arcane Save 2-4..than weigh possible single lvl dips vs. Lvl 3 and 4 of AS...or vs. Synth18/AS2

The only thing my synthesist would miss out on is Summon Monster 9 and Gate but really summons are only back-ups since I can't summon them when I'm usiong my Eidolon

Oh another one could be Monk 1 since your considered unarmored when fused with your eidolon...with a decent Wisdom (which can be fit since your dumping your 3 physical stats) Stunning fist could be quite useful to apply to a claw attack as well as flurry of blows...not sure how the 1D6 unarmed damage would function though. Oh and if I read it right Monks use both Wis and Dex for AC....The Monk level would actually fit quite well since Alphonse is an effective unarmed fighter....oh oh actually if I took the Monk of the Four Winds Archetype I could get "Elemental Fist" instead of Stunning fist allowing me to add 1D6 elemental damage to any unarmed attack(acid, cold, electricity, or fire...1/Day + 1 per 4 lvls)...which for an Eidolon using natural attack should be all of them? mixing Alphonse Unarmed combat prowess with Edwards Magical abilities.

Oh and of course 2 Paladin for the MASSIVE save boost but it doesn't fit AT ALL so thats out.