sickly sweet is an outcast changeling. like all changelings, she is able to produce a sticky green gel / slime, but she has a condition where she secretes it constantly and uncontrollably. in her changeling form, the slime is lime-green- but whenever she takes the form of a pony, the slime takes the color of her coat ( so it kind of looks like shes melting ). on some days she starts leaking slime like a faucet and is forced to hide from the public until her condition lessens. she was thrown out of her swarm because she was unable to fit in with the other changelings, and the slime she secreted often prevented her from doing important tasks.
 
she doesn’t really have a name, but other ponies call her ‘sickly sweet’ because she reminds them of melting ice cream that nopony would want to eat ( and, as you know, if ice cream is warm and melty and really sweet its not always that great ).  
she adopted the pony form by looking through a discarded recipe book for ice cream. she took the appearance of the character that recited the instructions inside of it. the cutie mark she has is an ice cream cone with three scoops of ice cream.
 
sickly sweet is highly self-conscious of condition and dislikes it when other ponies judge her solely based on her appearance. she tends to frighten away many ponies because of the slime she produces, but has managed to gain several friends that were willing to give her a second chance, but she has no intimate relationships with any of them. shes fairly mild-mannered . she tends to make a lot of social mistakes because she wasnt raised to learn how to behave properly in public.
 
because sickly sweet frightens so many ponies away, its difficult for her to feed off love, so she’s almost always hungry for it. she manages to get along by eating normal pony food but it doesnt completely satisfy her.  
her friends teach her to learn how to read, but being a changeling, she didnt know how to read- naturally, she has a difficulty learning it.  
she tries to substitute the need to feed on love by reading simple picture books about friendship and affection, because romance novels are too complex for her to decipher.