Well, I surpassed the character limit trying to send it to Arcturus, but here it is anyway:
Name: Lilith Ernestine
Appearance before merge: Long red hair, blue eyes, typically wore black, white, and blue clothes
Appearance after merge: (The merge made her taller and more muscular, which explains why she looks older)
Personality: Lilith is passive-aggressive, sarcastic, and occasionally mischievous. It’s a bit uncommon for her to be in a good mood and she can be VERY resentful. It will take a while before she reveals her more gentle and caring nature. When in battle, she is reckless and dangerous, but often stops herself before she passes her limits. However, if you hurt someone she cares about, she’ll go all out on you (due to the fact that her frontal lobe was slightly damaged in the merging process). She becomes an unstoppable, but not invincible, beast of rage and revenge. Even if you manage to escape her, she’ll stop at nothing until she gets her revenge on you for whatever you did.
History: Lilith was actually one of the earlier merges (which is why she isn’t merged with multiple Pokémon). Frightened, she had broken her cell open, accidentally killing some scientists with Night Daze. She had managed to evade the hunters for a while and was too scared to try to find out what the project was trying to do. So instead, she had returned to her home in Striaton City to hopefully find that her family was alright. She had found an empty house, but not too far away was her father, who ran when she saw him. Lilith chased the man down an alley, where he had revealed the truth to her. He told Lilith that he and her mother (which was killed in her escape) were part of the Project for many years and that she was planned to be merged before she was born (unlike in scattered shards, he actually regrets turning his daughter into a merge). In a sudden rage, she was almost about to kill him, then he sent out a Gardevoir and teleported away. Lilith still seeks her revenge on him.
Flaw: Lilith has uncontrollable aggression issues that she can never stop by herself, thus making her a threat to both enemies and allies. She also may be disliked by the other merges due to her passive aggressive and sarcastic nature. Also, I was reading some previous RPs she was in and realized there was a lot of moments that could have made her sanity shatter, so why not bring that element back? She has often gotten nightmares since the merge and wakes up completely restless because of it. Sometimes her subconscious also takes over her illusion abilities, and makes false objects or people that she mistakes for the real thing.
Other: Psychic types, please don't go into her mind. Just don't do it... ever...
Theme song: http://www.youtube.com/watch?v=S23-IcyBDDM
Battle Theme: https://www.youtube.com/watch?v=wKuvfgcPyh0 (The English lyrics fit her perfectly)