There is a deep level of customization available in Shin Megami Tensei IV through the Fusion system. It's relatively straight forward given that you're provided with 3 options to choose from, but in order to get the most preferable combination of skills and stats, you'll need to find the right combination of demons to fuse.
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Hamsa 3 Avian Law Gun: Weak, Elec: Resist - Phys 2 2 Angel 3 Divine Law Light: Null, Dark: Weak Bind: Resist Phys 1 1 Heqet 3 Holy Neutral Fire: Weak, Elec: Resist Poison: Null Phys 1 1 Pele 6 Lady Chaos Fire: Null, Ice: Weak - Phys 1 1 Erthys 8 Element Neutral Elec: Drain, Force: Weak, Light: Null, Dark: Null - Phys 1 1
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Hamsa 91 40 9 9 9 11 9 Zio, Attack Knowhow Dia 4 - - Angel 61 58 9 9 11 8 10 Dia, Patra Rakukaja 5 - - Heqet 91 40 9 9 9 9 9 Healing Knowhow, Zio Posumudi 4 - - Pele 76 72 11 12 15 11 14 Agi, Posumdi Healing Knowhow 8 - - Erthys 171 34 17 16 13 14 12 Elec Pleroma Domina 9 Life Bonus 12
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Fortuna 11 Megami Law Force: Drain - Phys 1 1 Suparna 15 Avian Law Elec: Null, Light: Resist, Dark: Weak Bind: Resist Phys 1 1 Shiisaa 14 Avatar Neutral Fire: Null, Elec: Resist, Force: Resist, Light: Null, Dark: Weak - Phys 1 1 Tam Lin 13 Genma Neutral Ice: Weak, Light: Null Sick: Weak Phys 1 2 Makara 13 Dragon Chaos Ice: Drain - Phys 1 1 Morax 15 Tyrant Chaos Fire: Repel, Ice: Weak, Dark: Null Sick: Weak Phys 1 1 Aeros 12 Element Neutral Elec: Weak, Force: Drain, Light: Null, Dark: Null - Phys 1 1
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Fortuna 147 66 17 16 16 20 17 Zan, Sukukaja Media 12 - - Suparna 175 77 21 20 20 25 21 Rakukaja, Media Spirit Drain 16 - - Shiisaa 227 45 25 22 18 20 17 Mazan, Counter Force Pleroma 16 - - Tam Lin 218 43 23 21 17 19 16 Hama, Gram Slice Maragi 14 Estoma Sword 16 Makara 161 72 19 18 18 23 19 Bufu, Zio Tarunda 14 Attack Knowhow 15 Morax 237 46 26 23 19 21 18 Oni-Kagura, Counter Life Bonus 16 - - Aeros 154 68 18 17 17 21 18 Force Pleroma Shibaboo 13 Mana Bonus 16
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Hathor 18 Megami Law Elec: Weak, Force: Resist, Light: Resist - Phys 1 1 Daphne 19 Tree Law Fire: Weak Panic: Resist, Bind: Null Phys 1 1 Archangel 18 Divine Law Light: Null, Dark: Weak - Phys 1 1 Apis 18 Holy Neutral Dark: Weak Panic: Resist Phys 1 1 Ictinike 18 Genma Neutral Fire: Drain, Force: Weak, Light: Null Sick: Weak Phys 1 1 Dzelarhons 17 Lady Chaos Fire: Repel, Ice: Weak, Dark: Null - Phys 1 1 Flaemis 20 Element Neutral Fire: Drain, Ice: Weak, Light: Null, Dark: Null - Phys 1 1 Aquans 16 Element Neutral Fire: Weak, Ice: Drain, Light: Null, Dark: Null - Phys 1 1
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Hathor 196 88 24 22 22 29 24 Spring of Life, Patra Posumudi 19 Rakunda 20 Daphne 137 131 22 24 31 21 29 Attack Knowhow, Rakukaja Resist Phys 21 - - Archangel 265 52 30 27 21 24 20 Hama, Heat Wave Tarukaja 19 Estoma Sword 20 Apis 265 52 30 27 21 24 20 Tetraja Media 19 Mazio 20 Ictinike 265 52 30 27 21 24 20 Agi, Damascus Claw Agilao 19 Speed Lesson 21 Dzelarhons 127 122 20 22 28 19 26 Agilao Hard Worker 19 Mana Bonus 20 Flaemis 210 94 26 26 26 26 26 Fire Pleroma Pulinpa 21 Mana Bonus 24 Aquans 122 117 20 22 27 19 25 Ice Pleroma Poisma 17 Life Bonus 20
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Parvati 25 Megami Law Fire: Weak, Ice: Drain, Light: Null, Dark: Null - Phys 1 1 Phoenix 25 Avian Law Fire: Drain, Ice: Weak, Light: Resist - Phys 1 1 Ame no Uzume 22 Amatsu Law Gun: Weak, Elec: Weak, Light: Null - Phys 1 1 Kamapua'a 24 Avatar Neutral Fire: Resist, Ice: Weak, Elec: Resist, Force: Weak Poison: Resist, Sleep: Weak Phys 1 1 Nata Taishi 24 Genma Neutral Gun: Resist, Light: Resist, Dark: Resist Poison: Resist Phys 1 1 Patrimpas 23 Dragon Chaos Ice: Resist, Force: Repel, Light: Weak, Dark: Weak - Phys 1 1 Take-Minakata 21 Kunitsu Chaos Fire: Weak, Elec: Drain, Light: Resist, Dark: Resist Bind: Weak Phys 1 1 Gnome 24 Element Neutral Elec: Repel, Force: Weak, Light: Null, Dark: Null - Phys 1 1 Ogun 24 Zealot Neutral Fire: Null - Phys 1 1 Mithras 21 Tyrant Chaos Fire: Resist, Ice: Weak, Light: Null - Phys 1 1
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Parvati 165 158 27 30 38 26 36 Media, Rakukaja Chakra Walk 26 - - Phoenix 245 109 31 29 29 37 31 Fire Breath, Recarm Fire Pleroma 27 - - Ame no Uzume 151 145 25 27 35 23 32 Sabbatma, Light Mana Aid Dekunda 24 Spring of Life 25 Kamapua'a 238 107 30 30 30 30 30 Dekaja, Patra Tarunda 25 Posumudi 26 Nata Taishi 322 63 37 33 26 30 25 Oni-Kagura, Resist Phys Dia 25 Charge 26 Patrimpas 155 149 26 28 36 24 33 Zanma, Mazan Recarm 24 Tetraja 25 Take-Minakata 293 58 33 30 24 27 23 Counter, Zionga War Cry 23 Elec Pleroma 25 Gnome 322 63 37 33 26 30 25 High Elec Pleroma Lullaby 25 Life Gain 28 Ogun 322 63 37 33 26 30 25 Blight, Tathium Shot Posumudi 25 Null Nerve 26 Mithras 293 58 33 30 24 27 23 Mahama, Heat Wave Rakunda 22 Fatal Sword 23
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Narcissus 28 Tree Law Fire: Weak, Force: Drain Panic: Resist Phys 1 1 Principality 26 Divine Law Gun: Weak, Light: Null, Dark: Weak - Phys 1 1 Thoth 30 Deity Neutral Gun: Weak, Fire: Null, Ice: Null, Dark: Weak - Phys 1 1 Pabilsag 26 Holy Neutral Gun: Repel, Elec: Weak Poison: Null Phys 1 1 Baldur 29 Genma Neutral Phys: Resist, Gun: Weak, Elec: Null - Phys 1 1 High Pixie 28 Fairy Neutral Force: Resist - Phys 1 1 Sedna 27 Lady Chaos Fire: Weak, Ice: Drain, Light: Weak, Dark: Null - Phys 1 1 Sylph 28 Element Neutral Elec: Weak, Force: Repel, Light: Null, Dark: Null - Phys 1 1 Kanbari 27 Enigma Neutral Phys: Weak, Gun: Null, Light: Null, Dark: Weak Bind: Weak Phys 1 1
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Narcissus 266 118 34 34 34 34 34 Panic Caster, Axel Claw Null Mind 29 Bufula 30 Principality 169 163 28 31 40 27 37 Dekunda, Posumudi Estoma Sword 27 Media 28 Thoth 188 181 32 35 45 30 41 Megido Mana Bonus 31 Doping 33 Pabilsag 341 67 40 35 28 32 27 Toxic Sting, Posumudi Blast Arrow 28 - - Baldur 273 121 35 32 32 42 35 Hamaon, Dream Fist Resist Light 30 Ally Counter 32 High Pixie 179 172 34 31 31 41 34 Mazanma, Trafuri Healing Knowhow 29 Force Pleroma 31 Sedna 350 68 41 36 29 33 27 Mabufula, Mamudo Posumudi 29 Diarama 31 Sylph 266 118 34 31 31 41 34 High Force Pleroma Bind Voice 29 Mana Gain 32 Kanbari 350 68 41 36 29 33 27 Hama, Rapid Needle Tetrakarn 30 - -
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Lailah 35 Herald Law Elec: Weak, Light: Null - Phys 1 1 Brigid 33 Megami Law Fire: Drain, Ice: Weak - Phys 1 1 Vidofnir 33 Avian Law Gun: Weak, Light: Null - Phys 2 2 Mayahuel 35 Tree Law Gun: Null, Fire: Weak - Phys 1 1 Power 33 Divine Law Force: Weak, Light: Null, Dark: Weak - Phys 1 1 Inti 35 Deity Neutral Fire: Null, Ice: Null, Elec: Weak - Phys 1 1 Makami 32 Avatar Neutral Light: Null Panic: Weak Phys 1 1 Qing Niugai 33 Holy Neutral Phys: Resist Panic: Null Phys 1 1 Frost Ace 34 Genma Neutral Fire: Repel, Ice: Repel - Phys 1 1 Minotaur 33 Beast Neutral Fire: Null, Ice: Weak, Dark: Null - Phys 1 1 Peallaidh 33 Wilder Neutral Fire: Null, Ice: Weak, Dark: Null Poison: Resist Phys 1 1 Black Frost 34 Jaki Neutral Phys: Resist, Fire: Repel, Ice: Drain, Dark: Null - Phys 1 1 Ares 31 Fury Chaos Light: Null, Dark: Weak Panic: Weak Phys 1 1 Gucumatz 33 Dragon Chaos Fire: Weak, Ice: Drain, Force: Resist, Dark: Weak - Phys 1 1 Okuninushi 34 Kunitsu Chaos Fire: Repel, Ice: Repel - Phys 2 2 Medusa 35 Femme Chaos Gun: Resist, Elec: Drain, Force: Weak - Phys 1 3 Lilim 33 Night Chaos Ice: Weak, Force: Null Sleep: Resist Phys 1 1 Undine 32 Element Neutral Fire: Weak, Ice: Repel, Light: Null, Dark: Null - Phys 1 1
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Lailah 212 205 36 40 51 34 47 Recarm, Mahama Hamaon 36 - - Brigid 301 135 39 39 39 39 39 Mediarama, Agilao Mana Gain 34 - - Vidofnir 202 196 34 38 48 32 45 Damascus Claw, Ally Counter Mazionga 34 Elec Pleroma 35 Mayahuel 212 205 36 40 51 34 47 Media, Me Patra Heal Pleroma 36 High Heal Pleroma 37 Power 407 81 48 43 34 39 32 Tathlum Shot, Diarama Megido 35 - - Inti 212 205 36 40 51 34 47 Maragion, Fire Pleroma Mabufula 36 Ice Pleroma 37 Makami 294 131 38 35 35 45 38 Tetrakarn, Null Mind Posumudi 33 Workaholic 35 Qing Niugai 407 81 48 43 34 39 32 Rakukaja, Megaton Press Silent Prayer 34 - - Frost Ace 308 138 40 37 37 48 40 Mabufula, Ice Pleroma Purple Smoke 35 - - Minotaur 407 81 48 43 34 39 32 Oni-Kagura, Phys Pleroma Charge 34 Ally Counter 35 Peallaidh 202 196 34 38 48 32 45 Axel Claw, Poison Breath, Pulinpa Panic Voice 34 Beastly Reaction 35 Black Frost 388 76 46 41 32 37 31 Ice Breath, Taunt Mamudo 32 Endure 34 Ares 388 76 46 41 32 37 31 Fatal Sword, Counter Tetrakarn 33 - - Gucumatz 301 135 39 36 36 47 39 Rakunda, Megido Draconic Reaction 35 Resist Fire 37 Okuninushi 416 82 50 44 35 40 33 Dream Fist, Healing Knowhow Diarama 36 Estoma Sword 38 Medusa 315 140 41 41 41 41 41 Stun Needles, Mazionga Tathlum Shot 36 Gun Pleroma 38 Lilim 202 196 34 38 48 32 45 Dream Needle, Mana Gain Mazan 34 Null Gun 36 Undine 197 190 33 37 47 31 44 High Ice Pleroma Poison Breath 33 Mana Gain 35