Pokémon: Rebecca Anderson (Grounded)
Type: Normal
Ability: Long Reach
Signature Move: Mercurius Morph - Normal, Special, 60 BP, 100% Acc, 10 PP | If used by Rebecca Anderson or Demon Rebecca, changes her form from Grounded to Airborne or vice versa (like Meloetta's Relic Song). In the latter case, her Speed stage is reset to 0, but this move gains 20 BP per Speed stage above 0.
Moves: Pin Missile, Hyper Beam, Magical Leaf, Solar Beam, Fire Spin, Rock Slide, Icy Wind, Will-O-Wisp, Swift, Weather Ball, Rock Tomb, Bullet Seed, Icicle Spear, Mud Shot, Rock Blast, Brine, Avalanche, Psycho Cut, Venoshock, Electroweb, Air Slash, Flamethrower, Hydro Pump, Surf, Ice Beam, Blizzard, Thunderbolt, Thunder, Fire Blast, Tri Attack, Sludge Bomb, Spikes, Shadow Ball, Heat Wave, Muddy Water, Toxic Spikes, Aura Sphere, Seed Bomb, Dark Pulse, Dragon Pulse, Power Gem, Focus Blast, Energy Ball, Flash Cannon, Leaf Storm, Gunk Shot, Stealth Rock, Sludge Wave, Electro Ball, Scald, Hurricane, Dazzling Gleam, Smack Down, Infestation, Shock Wave, Water Pulse, Incinerate, Double-Edge, Extreme Speed, Quick Attack, Dragon Ascent, Fly, Sucker Punch, Pursuit, U-turn, Defog, Detect, Gen 5+ universal TMs
Stats: 76/112/68/123/87/54 | 520 BST
Form: Rebecca Anderson (Airborne)
Type: Normal/Flying
Ability: Speed Boost
Stats: 76/97/78/103/82/130 | 566 BST (-15 Atk, +10 Def, -20 SpA, -5 SpD, +76 Spe)
Mega Evolution: Demon Rebecca (Grounded)
Stone: Rebecca's Demon Crystal
Type: Dark/Normal
Ability: Long Reach
Stats: 76/138/107/143/94/62 | 620 BST (+26 Atk, +39 Def, +20 SpA, +7 SpD, +8 Spe)
Form: Demon Rebecca (Airborne)
Type: Dark/Flying
Ability: Drawing Dead (from Saffron)
Stats: 76/123/117/123/89/138 | 666 BST
Note: Airborne Rebecca Anderson can Mega Evolve into Airborne Demon Rebecca. (Think M4A Mimikyu.)
(Also, the blue-haired girl is a separate character named Shar.)
Reasoning: Cinque's childhood friend, nicknamed Becky, becomes the Hero of Pastillage after Nanami joins the fray. Accordingly, she gains the sacred "sword" Mercurius, which she transforms into either a sniper rifle or a rocket broom. This suits her best because she is not athletic like the other two heroes (so Normal makes sense as her primary type), but she is something of an FPS junkie.
Most of her movepool is inspired by Bullet Cards, in which crystalline energy is stored to be unleashed to a destructive effect. Namely, she gets TMs/TRs of projectiles (and similar) that damage others, in addition to Drawing Dead as an airborne demon.
The rest comes from the rocket broom form of Mercurius, and Long Reach in her grounded forms allows the moves to make sense even with the sniper rifle. In particular, Dragon Ascent goes by the following description: "After soaring upward, the user attacks its target by dropping out of the sky at high speeds[...]", while Sucker Punch (JP: Surprise Attack) is a possible description of the air raids in which Pastillage specializes. The rocket broom also gives Flying typing, just as Celesteela and Mega Maple have it. (Speed Boost as well, in the latter vein.)
Before her first departure from Flonyard (Cinque's second), the Demon Lord Valério creates a Demon Crystal for her, allowing her to temporarily transform into an adult as Cinque and Gaul do with their Hero Crystals. (Houndoom supports the association of demons with Dark typing.) She loses Speed Boost in her airborne demon form because she uses the Demon Crystal purely to gain power.