What if we gave him Persoan's required for the Empress link and a couple others here and there.
This would give him
Floor 1:
Izanagi
Resist-Elec, Nulll- Dark, Weak-Wind
Skills: Zio, Cleave, Rakukaja, Tarukaja,Rakunda
Stats: Average in everything
Ippon Datara
Resist-Fire, Null-Dark, Weak-Light
Skills: Mudo, Maragi, Mamudo, Agilao, Rampage, Mudo Boost, Sukukaja
Stats: High Strength and Endurance, low magic
Floor 2: Matador
Repel-Dark, Weak-Fire
Skills: Mamudo, Swift Strike, Dekunda, Mudo Boost, Auto Suku, Rampage, Survive Light, Mahama
Stats: High Agility, low magic
Floor 3: Gdon
Absorb-Fire, Weak-Ice
Skills: Agilao, Maragi, Fire Break, Maragion, Evade Ice, Rampage, Fire Boost
Stats: High Strength, low magic
Floor 4: Neko Shogun
Resist-Physical, Null-Dark, Light, Reflect- Elec, Weak-Wind
Skills: Zionga, Elec Boost, Black Spot, Mediarama, Matarukaja, Evade Wind, Divine Grace, Bufula
Stats: High strength, low luck
Black Frost:
Absorb Fire and Ice, Reflect Darkness
Skills: Maragion, Bufula, Ice Boost, Mind Charge, Masukunda, Fire Amp, Agidyne, Mudoon
Stats: High Magic, average everywhere else.
Floor 5: Yatagasuru
Resist Wind, Null Light, Weak Ice
Skills: Masukukaja, Nervundi, Agilao, Null Fear, Diarahan, Cool Breeze, Megido
Stats: High Agility, Strength and Magic, low luck and endurance
Yatsufusa
Absorb Fire, Reflect Wind, Null Ice
Skills: Agidyne, Heatwave, Fire Boost, Masukukaja, Power Charge, Dodge Physical, Maragidyne, Mind Charge, Mediarama
Stats: High agility, low luck
Floor 6: Ganesha
Weak Elec, Null Wind
Skills: Tetrakarn, Garudyne, Mustard Bomb, Makarakarn, Power Charge, High Counter, Endure, Magarudyne.
Stats: High Strength, Low Agility
Floor 7: Trumpeter
Absorb Ice, Reflect Elec and Light, Null Darkness
Skills: Mind Charge, Megidola, Megidolaon, Heat Riser, Debilitate, Ziodyne, Elec Amp, Cool Breeze, Marakukaja
This set up leaves a couple of holes in Souji’s skillset but it seems like a pretty simple way to interpret him. A couple additional things we could do are let him pick one additional persona on two floors. It would be best if one choice was on an earlier floor and the other is on a later floor. For example on floor 3 or so he would get to pick any level 30 or under persona, and another on floor 8 level 80 or under. This would also work out nicely because he would end up with 12 personas. Additionally, I think with this set up it would be fair if Izanagi evolved if Souji reached floor 10.