Name: Sans, the Skelepun Pokémon
Type: Psychic/Normal
Ability: Bad Time: If sent out as the last non-fainted Pokémon on the team, it changes into Genocide Forme.
Stats: Laughably low stats. Very high HP.
Formes: Genocide Forme: If sent out as the last non-fainted Pokémon on the team, it changes into Genocide Sans.
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Name: Genocide Sans
Type: Psychic/Nornal
Ability: Vengence (Sheer Force Variant)
Stats: Massivly high balanced stats rivaling most legendaries.
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Name: Flowey, the Michevious Flower Pokémon
Type: Grass/Ghost
Ability: Shadow Tag/SAVE & LOAD: When an opponent is hit or knocked out, they are done twice, without added damage, activating move effects and Abilities effected twice (for example, Power-Up Punch gets +2 Attack instead of 1+ but only deals damage once, and Moxie gains +2 instead of +1. Enemy abilities don't count so Iron Thorns only hurts once.).
Stats: Balanced stats all around, except Sp. Attack, which is very low, but has high Speed.
Formes: Mega Evolution
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Item: Flowite
Name: Mega Flowey
Type: Grass/Ghost
Ability: Omega Guard (Magic Bounce Variant)
Stats: All stats except Speed, HP and Sp.Att are boosted, Attack boosted more then others.
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Name: Undyne, the Over-Determined Pokémon
Type: Water/Steel
Ability: True Hero: Raises Attack when it switches in. Changes Form if weakened (see below).
Stats: Large Attack power with balanced Defensive stats. Moderate HP. Low Sp.Attack. Very low Speed.
Formes: Undying Forme (part of its ability): If Undyne is to faint on a turn it didn't attack, it transforms, enduring the attack and if its using an Attack move, it deals double damage. Stats, Ability and such stay the same except its forme.
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Name: Papyrus, the Bone Guard Pokémon
Type: Psychic/Fire
Ability: Bonetrousle: Sp.Attack rises each turn
Stats: Very defensive, but not very offensive. Great HP. Very low Speed.
Formes: None
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Name: Alphys, the Lab Lizard Pokémon
Type: Psychic/Electric
Ability: Rattled/Intellectual (Simple Variant)
Stats: High HP and Sp. Defense with moderate Sp. Attack and Defense, and low Speed (like Clefable).
Formes: None
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Name: Toriel, the Motherly Pokémon
Type: Normal/Dragon
Ability: Telepathy/Royal Defense: The user is immune to all status conditions.
Stats: Good HP, Sp.Attack and Speed, great Sp.Defense but slightly worse Defense. Low Attack.
Formes: None
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Name: Asgore, the Kingly Pokémon
Type: Normal/Dragon
Ability: Telepathy/Royal Offense: Breaks through any and all barriers.
Stats: In comparison to Toriel's; has much higher Attack and Speed, much lower Sp.Defense but slightly higher Defense, and its SP.Attack is very low.
Formes: None
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Name: Mettaton, the Fabulous Robot Pokémon
Type: Steel
Ability: Unbreaking: Moves that make direct contact deal no damage and hurts the attacker.
Stats: Extremely high Defense, moderate HP and Attack, but low Sp.Attack and extremely low Speed.
Formes: Mettaton EX: The user transforms by hitting the switch behind it, which can be done outside of battle.
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Name: Mettaton EX
Type: Steel/Fairy
Ability: Star Of The Show: When an opponent is Attracted, the user gains double Attack until the infatuation ends.
Stats: Sp. Defense and Attack are raised, Defense is brought down significantly, but Speed is massively boosted. Sp. Attack remains the same.
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Name: Dummy, the Dummy Pokémon
Type: Normal/Ghost
Ability: Determine: The user is immune to every form of damage, but only for the first turn. It stalls on the first turn, then transforms depending on the outcome. If it switches in, it will still count that as the first turn. The user's ability cannot be removed or copied until it transforms.
Stats: 1 in every stat.
Formes: Dummy has two Formes
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Name: Mad Dummy: Hitting the regular Dummy will transform it into Mad Dummy.
Type: Fire/Ghost
Ability: Aggressive Spirit: Can only Attack. In turn, it's Attacks can hit Pokémon who would normally be immune to it (like ghost hitting normal, fire hitting flash fire).
Stats: Stats are now majorally buffed Has high Attack, Sp.Attack and Speed. Has low Defense and Sp. Defense.
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Name: Glad Dummy: Not hitting the regular Dummy will transform it into Glad Dummy
Type: Water/Ghost
Ability: Passive Spirit: Can only use Status Moves. In turn, it can cause a status condition to any Pokémon no matter if they are immune to it (like paralyzing limber or poisoning steel).
Stats: Stats are now majorally buffed. Has high Defense, Sp. Defense and Speed but low Attack, Sp. Attack.
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I'll do the others later, including Muffet, Gaster and Frisk.