Krystal grew up in a home on the outskirts of a nearby village. She had a loving mother and father who raised her with as much love as anybody could give a child.
In the Syllagust family, there is a recessive gene that causes one or both ears to droop, and partial/full deafness occurs. Krystal's right ear droops, and she's almost entirely deaf in it.
Other kids always picked on her defective ear and always came up with hurtful names. When she got a little older, she started telling people her ideas, and thusly she was picked on for those too. She thought her ideas for a better world were right, and not until she met Dr. Robotnik did she ind anyone who understood her.
When she met Eggman, he had told her that all her ideas were great, and tricked her into thinking that he was good and Sonic and his friends were evil. (obviously she must have lived in a shell, am I right?) He didn't entirely trust her loyalty to him, so to ensure that she didn't tell everyone his plans or turn good, he placed a circular device on her back that monitered where her loyalties lie, and if she was turning the other way, it would shock her until she fell in place.
He sent her to spy on Sonic and his friends, acting as if she were one of them
Relationships with other characters
Sonic the hedgehog: She doesn't much care for him, and despises his way of thinking
Shadow the hedgehog: Because of their common encounters with Eggman, and his dark demeanor, she has a love interest in him.
Eggman: She works for him as if she's one of his common lackeys.
Rouge and Amy Rose: She has a rivalry with both of them, mosty because she views them as being "whores". She also doesn't like Amy because she's a main part of Sonic's crowd, and she doesn't like Rouge because of how close she is to Shadow. Rouge has mutual feelings towards Krystal.
Espio the chameleon: She has taken quite a liking to him, and they could have the potential to have a love interest. She's quite good friends with him.
Liwi the fox: Liwi as Krystal's first love, and also helped her figure out that she was also interested in women. She still remains friends with Liwi
Nugget the hybrid: They met while they were in school. She fell head over heels for him, and let him do whatever he wanted to do to her. He never truly loved her until after he lost her. While they were dating, he claimed that at a party, she got drunk and was raped, and that's how she ended up getting pregnant with Terios. They are somewhat friends now.
Flea the hedgehog: He had found her injured on one of his excursions, and it was basically lust at first sight. He nursed her back to health, and afterwards they were married for several years. He fathered her two other children, Skittle and Vincent. They got divorced after Krystal found him cheating on her.
Shadow the hedgehog: Before she met Flea, she had a small fling with him, but after the divorce, they became more romantically involved. He treats her three children as if they were his own.
Serena-Lola (Lelly): In an alternate universe, after the divorce with Flea, Krystal falls for her. It's not quite a serious relationship but they really do care for each other.
Warning: this contains spoilers
After serving Eggman for many years and giving valuable information about Sonic and his friends, Krystal is set free from the loyalty device after Shadow breaks it off her. Krystal remains an anti-hero, but she no longer sides with him. It becomes an all-out war between Sonic and Eggman. She does find out who Terios's dad is, eventually.
Later, Sonic bcomes king like he does in the comics. Shadow and Krystal end up married. He sets up the Knothole Royal Guard, and all his friends, including Shadow, Krystal, and Terios, are in it. Krystal ends up not being able to fight one day because of an illness, and Shadow tries to stay with her, but to no avail. He gets sent off to fight the highly evolved mecha-bots that Eggman has sent to guard him. Shadow nearly gets killed in the battle, but when news reaches her, she's been told that he was killed in battle. She becomes over come with grief, and is beside herself with joy when he comes back.
Eggman's empire had fallen, and Mobius is a safe place for all once again.
Krystal becomes the grandmother of nine grandchildren.
Krystal attacks using her claws and her scythe. She is a speed character and is on a team (as of yet, unnamed)
Orignally, she had much more children than the ones listed. From Shadow, she had Terios, Truth, Pheonix, Maria, and Roxie, and from Flea, she had Carmello, Skittle, Kayla, Summer, Autumn, and Vincent. She also had Steve from Tails and Kimora from Sonic. Only three now exist.
She has a lesbian anthro form named "Crystal", whom is hardly ever used.